Liquid crystal display/optical retardation compensator combination in which variations in the dispersion of light in the liquid crystal and/or in the compensator materials minimize undesired screen coloration

ABSTRACT

The liquid crystal display device is composed of at least one optical retardation compensator plate  2  (and  3 ) inserted between a liquid crystal display element  1  and polarizer plates  4  and  5 . The liquid crystal display element  1  is composed of a pair of electrode substrates  6  and  7  and a liquid crystal layer  8  sealed therebetween. The polarizer plates  4  and  5  flank the liquid crystal display element  1 . The optical retardation compensator plate  2  (and  3 ) has a negative refractive index anisotropy (na=nc&gt;nb). The direction of a principal refractive index nb parallel to the normal to the surface and the direction of either a principal refractive index na or nc in the surface incline either clockwise or counterclockwise around the direction of the principal refractive index nc or na in the surface. Moreover, either the pretilt angle formed by the orientation films  11  and  14  and the longer axes of liquid crystal molecules in the liquid crystal layer or the value of applied voltage for displaying halftone obtained by applying to the liquid crystal a voltage that is close to the threshold voltage for the liquid crystal is set within such a range that tone reversion does not occur in the opposite viewing direction when halftone is being displayed.

This application is a continuation of U.S. patent application Ser. No.09/597,520, filed 20 Jun. 2000 now U.S. Pat. No. 6,535,258, that in turnis a division of U.S. patent application Ser. No. 09/056,035, filed 6Apr. 1998, now U.S. Pat. No. 6,137,556, issued 24 Oct. 2000.

FIELD OF THE INVENTION

The present invention relates to a liquid crystal display device,especially, to a liquid crystal display device with the viewing angledependency of the display screen abated by a combination of a liquidcrystal display element and an optical retardation compensator plate.

BACKGROUND OF THE INVENTION

Conventionally, liquid crystal display devices incorporating nematicliquid crystal display elements have been in widespread use fornumeral-segment-type display devices such as watches and calculators,and recently the applications are finding more places with wordprocessors, notebook-type personal computers, liquid crystal televisionsmounted in automobiles, etc.

Generally, a liquid crystal display element has a transparent substrate,electrode lines for turning on and off pixels, and other componentsformed on the substrate. For example, in an active-matrix type liquidcrystal display device, active elements, such as thin-film transistors,are formed on the substrate together with the electrode lines asswitching means for selectively driving pixel electrodes by whichvoltages are applied across the liquid crystal. Moreover, in liquidcrystal display devices capable of color display, color filter layershaving colors such as red, green and blue are provided on the substrate.

Liquid crystal display elements such as the one mentioned above adopt aliquid crystal display mode that is suitably selected depending on thetwist angle of the liquid crystal: some of well-known modes areactive-driving-type twisted nematic liquid crystal display mode(hereinafter, referred to as the TN mode) and the multiplex-driving-typesuper-twisted nematic liquid crystal display mode (hereinafter, referredto as the STN mode).

The TN mode displays images by orientating the nematic liquid crystalmolecules to a 90°-twisted state so as to direct rays along the twisteddirections. The STN mode utilizes the fact that the transmittance isallowed to change abruptly in the vicinity of the threshold value of theapplied voltage across the liquid crystal by expanding the twist angleof the nematic liquid crystal molecules to not less than 90°.

The problem with the STN mode is that the background of the displayscreen sustains a peculiar color due to interference between colorsbecause of the use of the birefringence effect of liquid crystal. Inorder to solve this problem and to provide a proper black-and-whitedisplay in the STN mode, the application of an optical retardationcompensator plate is considered to be effective. Display modes using theoptical retardation compensator plate are mainly classified into twomodes, that is, the double layered super-twisted nematicoptical-retardation compensation mode (hereinafter, referred to as theDSTN mode) and the film-type optical-retardation compensation mode(hereinafter, referred to as the film-addition mode) wherein a filmhaving optical anisotropy is provided.

The DSTN mode uses a two-layered construction that has a display-useliquid crystal cell and a liquid crystal cell which are orientated witha twist angle in a direction opposite to that of the display-use liquidcrystal cell. The film-addition mode uses a construction wherein a filmhaving optical anisotropy is disposed. Here, the film-addition mode isconsidered to be more prospective in the standpoint of light weight andlow costs. Since the application of such an optical-retardationcompensation mode makes it possible to improve black-and-white displaycharacteristics, color STN liquid crystal display devices have beenachieved that enable color display by installing color-filter layers inSTN-mode display devices.

The TN modes are, on the other hand, classified into the Normally Blackmode and the Normally White mode. In the Normally Black mode, a pair ofpolarizer plates are placed with their polarization directions inparallel with each other, and black display is provided in a state whereno ON voltage is applied across the liquid crystal layer (OFF state). Inthe Normally White mode, a pair of polarizer plates are placed withtheir polarization directions orthogonal to each other, and whitedisplay is provided in the OFF state. Here, the Normally White mode isconsidered to be more prospective from the standpoints of displaycontrast, color reproducibility, viewing angle dependency, etc.

However, in the TN-mode liquid crystal display device, liquid crystalmolecules have a refractive index anisotropy An, and are orientated soas to incline to the above and below substrates. For these reasons, theviewing angle dependency increases: i.e., the contrast of displayedimages varies depending upon the direction and angle of the viewer.

FIG. 11 schematically shows the cross-sectional construction of a TNliquid crystal display element 31. This state shows liquid crystalmolecules 32 slanting upward slightly as a result of application of avoltage for halftone display. In such a liquid crystal display element31, a linearly polarized ray 35 passing through the surfaces of a pairof substrates 33 and 34 along the normals thereto, and linearlypolarized rays 36 and 37 passing through those surfaces not along thenormals thereto cross the liquid crystal molecules 32 at differentangles. Besides, the liquid crystal molecules 32 have a refractive indexanisotropy An. Therefore, the linearly polarized rays 35, 36 and 37,upon passing through the liquid crystal molecules 32 in differentdirections, produce ordinary and extraordinary rays. The linearlypolarized rays 35, 36 and 37 are converted to elliptically polarizedrays according to the phase difference between the ordinary andextraordinary rays, which cause the viewing angle dependency.

In addition, in an actual liquid crystal layer, the liquid crystalmolecules 32 show different tilt angles in the vicinity of the midpointbetween the substrates 33 and 34 and in the vicinities of the substrates33 and 34. The liquid crystal molecules 32 are twisted by 90° around thenormal.

For those reasons described so far, the linearly polarized rays 35, 36and 37 passing through the liquid crystal layer are affected by thebirefringence effect in various ways depending upon, for example, thedirections and the angles thereof, resulting in complex viewing angledependency.

Such viewing angle dependency can be observed, as examples, in thefollowing situations. If the viewing angle increases from the normal tothe display screen in the standard viewing direction, i.e. downward, andexceeds a certain angle, the displayed image has a distinct color(hereinafter, referred to as the coloration phenomenon), or is reversedin black and white (hereinafter, referred to as the tone reversionphenomenon). If the viewing angle increases from the normal in theopposite viewing direction, i.e. upward, the contrast decreasesabruptly.

The aforementioned liquid crystal display device has another problemthat the effectual range of viewing angle narrows with a larger displayscreen. When a large liquid crystal display device is viewed from ashort distance in the front thereof, the same color may appear differentin the uppermost and lowermost parts of the large screen due to theeffect of the viewing angle dependency. This is caused by a wider rangeof viewing angle required to encompass the whole screen surface, whichis equivalent to a viewing direction which is increasingly far offcenter.

To restrain the viewing angle dependency, Japanese Laid-Open PatentApplications No. 55-600/1980 (Tokukaisho 55-600) and No. 56-97318/1981(Tokukaisho 56-97318) suggest that an optical retardation compensatorplate (retardation compensator film) be inserted as an optical elementhaving optical anisotropy between the liquid crystal display element andone of polarizer plates.

According to the method, the elliptically polarized ray converted from alinearly polarized ray by passing through liquid crystal moleculeshaving refractive index anisotropy is directed through the opticalretardation compensator plate(s) disposed on the side(s) of the liquidcrystal layer having refractive index anisotropy. Hence, the phasedifference between the ordinary and extraordinary rays which occurs tothe viewing angle are compensated for, and the elliptically polarizedray is converted back to the linearly polarized ray, which enables therestraint of the viewing angle dependency.

Japanese Laid-Open Patent Application No. 5-313159/1993 (Tokukaihei5-313159), as an example, discloses an optical retardation compensatorplate of the above kind represented by a refractive index ellipsoid withone of the principal refractive indices parallel to the normal to thesurface of the optical retardation compensator plate. Nevertheless, thisoptical retardation compensator plate still cannot satisfactorilyrestrain the tone reversion phenomenon that occurs when the viewingangle increases in the standard viewing direction.

In order to eliminate the tone reversion phenomenon, Japanese Laid-OpenPatent Application No. 57-186735/1982 (Tokukaisho 57-186835) disclosesthe so-called pixel dividing method, in which a displayed pattern(pixel) is divided and orientation is controlled so that each dividedsegment has its own viewing angle characteristics independent from thoseof the other segments. According to the method, since the liquid crystalmolecules stand upwards in different directions from segment to segment,the viewing angle dependency can be eliminated. However, the problem ofa lower contrast when the viewing angle increases upward or downwardcannot be solved.

Japanese Laid-Open Patent Applications No. 6-118406/1994 (Tokukaihei6-118406) and No. 6-194645/1994 (Tokukaihei 6-194645) disclosetechnologies to combine the pixel dividing method and an opticalretardation compensator plate.

The liquid crystal display device disclosed in Japanese Laid-Open PatentApplication No. 6-118406/1994 includes an optical anisotropic film(optical retardation compensator plate) interposed between the liquidcrystal panel and the polarizer plate to, for example, improve thecontrast. The retardation compensator plate (optical retardationcompensator plate) disclosed in Japanese Laid-Open Patent ApplicationNo. 6-194645/1994 is set to have almost no phase difference in a planeparallel to the surface of the retardation compensator plate and to havea larger refractive index in a plane perpendicular to the surface of theretardation compensator plate than the refractive index in a planeparallel thereto, in order to have a negative refractive index.Therefore, when a voltage is applied, the positive refractive indexoccurring to the liquid crystal display element is compensated for andviewing angle dependency can be decreased.

Nevertheless, the application of the pixel dividing method to the use ofthis optical retardation compensator plate still fails to uniformlyrestrain the decrease in contrast in the vertical directions; colorationphenomenon still occurs in oblique directions when the viewing angle is45°.

For these reasons, there are limits to the restraining of the contrastvariation, coloration phenomenon, and tone reversion phenomenon relatedwith viewing angle, by means of a retardation compensator platerepresented by a refractive index ellipsoid positioned upright, i.e., arefractive index ellipsoid with one of the principal refractive indicesthereof parallel to the normal to the surface of the retardationcompensator plate.

Hence, Japanese Laid-Open Patent Application No. 6-75116/1994(Tokukaihei 6-75116) suggests the use of an optical retardationcompensator plate represented by a refractive index ellipsoid with theprincipal refractive indices inclining to the normal to the surface ofthe optical retardation compensator plate. This method adopts two kindsof optical retardation compensator plates as follows.

One of the optical retardation compensator plates can be represented bysuch a refractive index ellipsoid that the smallest of the threeprincipal refractive indices is parallel to the surface, one of the twolarger principal refractive indices inclines to the surface of theoptical retardation compensator plate by an angle θ, the remainingprincipal refractive index inclines to the normal to the opticalretardation compensator plate by the same angle θ, and the angle θsatisfies 20°≦θ≦70°.

The other optical retardation compensator plate can be represented by arefractive index ellipsoid inclining to the surface, where the threeprincipal refractive indices, na, nb, and nc, are mutually related bythe inequality na=nc>nb, and the direction of the principal refractiveindex nb parallel to the normal to the surface and the direction ofeither the principal refractive index na or nc in the surface reclineeither clockwise or counterclockwise around the direction of theprincipal refractive index nc or na in the surface.

As for the former optical retardation compensator plate, a uniaxial andbiaxial optical retardation compensator plate can be used. For thelatter one, two optical retardation compensator plates, instead of one,can be used in such a combination that the two principal refractiveindices nb form an angle of 90°.

A liquid crystal display device, incorporating at least one such opticalretardation compensator plate between the liquid crystal display elementand the polarizer plate exhibits some restraint in the contrastvariations, coloration phenomenon, and tone reversion phenomenon causedby the viewing angle dependency of the display screen.

However, with today's increasingly large demand on a wider effectualrange of viewing angle and superb display quality, a better restraint inthe viewing angle dependency is crucial. In this context, the opticalretardation compensator plate disclosed in Japanese Laid-Open PatentApplication No. 6-75116/1994 (Tokukaihei 6-75116) above does not providesatisfactory solutions and needs to be improved.

SUMMARY OF THE INVENTION

In view of the above problems, the first object of the present inventionis, on top of the improvement by the compensation effects by the opticalretardation compensator plate, to restrain the viewing angle dependency,and especially, to effectively restrain the tone reversion in theopposite viewing direction when halftone is being displayed by applyinga voltage that is close to the threshold voltage for the liquid crystal.

The second object of the present invention is, on top of the improvementby the compensation effects by the optical retardation compensatorplate, to restrain the viewing angle dependency, and especially, toeffectively restrain the coloration phenomenon.

In order to accomplish the first object, a liquid crystal display deviceof the first arrangement in accordance with the present inventionincludes:

a liquid crystal display element formed by sealing a liquid crystallayer between a pair of substrates;

a pair of polarizers disposed so as to flank the liquid crystal displayelement; and

at least one optical retardation compensator plate disposed between theliquid crystal display element and the polarizers, the opticalretardation compensator plate being represented by an incliningrefractive index ellipsoid,

wherein the pretilt angle formed by the orientation films and the longeraxes of liquid crystal molecules in the liquid crystal layer is setwithin such a range that tone reversion does not occur in the oppositeviewing direction when halftone is being displayed by applying to theliquid crystal a voltage that is close to the threshold voltage for theliquid crystal.

As explained above, the first arrangement of the present inventionincorporates, between the liquid crystal layer and the polarizer, anoptical retardation compensator plate represented by an incliningrefractive index ellipsoid. Therefore, with the arrangement, for a casewhere a linearly polarized ray is converted to an elliptically polarizedray according to the phase difference between the ordinary andextraordinary rays developed from the linearly polarized ray upon thepassing through the liquid crystal layer possessing birefringence, theoptical retardation compensator plate compensates for the phasedifference between the ordinary and extraordinary rays that variesdepending upon the viewing angle.

With the liquid crystal display device of the first arrangement inaccordance with the present invention, the pretilt angle of the liquidcrystal layer sealed in the liquid crystal display element is set withinsuch a range that tone reversion does not occur in the opposite viewingdirection when halftone is being displayed by applying to the liquidcrystal a voltage that is close to the threshold voltage for the liquidcrystal. This can eliminate the tone reversion in the opposite viewingdirection on a screen displaying halftone, and thereby further restrainthe viewing angle dependency of the screen. The contrast variations andcoloration are also restrained better than only by the compensationfunction by the optical retardation compensator plate.

In the first arrangement above, the abrupt decrease in luminance can berestrained in the standard viewing direction when halftone is beingdisplayed, by further setting the pretilt angle within such a range thatluminance does not decrease abruptly in the standard viewing directionwhen halftone is being displayed by applying to the liquid crystal avoltage that is close to the threshold voltage for the liquid crystal.

For these reasons, with the arrangement, the contrast ratio in black andwhite display is not affected by the viewing angle of the observer, andthe quality of images displayed by the liquid crystal display device isgreatly improved.

In order to accomplish the first object, a liquid crystal display deviceof the second arrangement in accordance with the present inventionincludes:

a liquid crystal display element formed by sealing a liquid crystallayer between a pair of substrates;

a pair of polarizers disposed so as to flank the liquid crystal displayelement; and

at least one optical retardation compensator plate disposed between theliquid crystal display element and the polarizers, the opticalretardation compensator plate being represented by an incliningrefractive index ellipsoid,

wherein the value of applied voltage for displaying halftone obtained byapplying to the liquid crystal a voltage that is close to the thresholdvoltage for the liquid crystal is set within such a range that tonereversion does not occur in the opposite viewing direction when halftoneis being displayed.

Even if a linearly polarized ray is converted to an ellipticallypolarized ray according to the phase difference between the ordinary andextraordinary rays developed from the linearly polarized ray upon thepassing through the liquid crystal layer possessing birefringence, thesecond arrangement compensates for the phase difference by the opticalretardation compensator plate similarly to the first arrangement.

With the liquid crystal display device of the second arrangement, thevalue of applied voltage for displaying halftone obtained by applying tothe liquid crystal a voltage that is close to the threshold voltage forthe liquid crystal is set within such a range that tone reversion doesnot occur in the opposite viewing direction when halftone is beingdisplayed. This can eliminate the tone reversion in the opposite viewingdirection with a screen displaying halftone, and thereby furtherrestrain the viewing angle dependency of the screen. The contrastvariations and coloration are also restrained better than only by thecompensation function by the optical retardation compensator plate.

In the second arrangement above, the abrupt decrease in luminance can berestrained in the standard viewing direction when halftone is beingdisplayed, by further setting the value of applied voltage fordisplaying halftone obtained by applying to the liquid crystal a voltagethat is close to the threshold voltage for the liquid crystal withinsuch a range that luminance does not decrease abruptly in the standardviewing direction when halftone is being displayed.

For these reasons, with the arrangement, the contrast ratio in black andwhite display is not affected by the viewing angle of the observer, andthe quality of images displayed by the liquid crystal display device isgreatly improved.

In order to accomplish the second object, a liquid crystal displaydevice of the third arrangement in accordance with the present inventionincludes:

a liquid crystal display element formed by sealing a liquid crystallayer between a pair of substrates;

a pair of polarizers disposed so as to flank the liquid crystal displayelement; and

at least one optical retardation compensator plate disposed between theliquid crystal display element and the polarizers, the opticalretardation compensator plate being represented by an incliningrefractive index ellipsoid,

wherein the ratios of the variation in the refractive index anisotropy,Δn_(L), of the liquid crystal material for the liquid crystal layer withthe wavelength of light and of the variation in the refractive indexanisotropy, Δn_(F), of the optical retardation compensator plate withthe wavelength of light are set within such a range that viewing angledependency does not cause coloration on the liquid crystal screen.

Even if a linearly polarized ray is converted to an ellipticallypolarized ray according to the phase difference between the ordinary andextraordinary rays developed from the linearly polarized ray upon thepassing through the liquid crystal layer possessing birefringence, thearrangement compensates for the phase difference by the opticalretardation compensator plate, similarly to the first arrangement.

With the liquid crystal display device of the third embodiment, theratios of the variation in the refractive index anisotropy, Δn_(L), ofthe liquid crystal material for the liquid crystal layer with thewavelength of light and of the variation in the refractive indexanisotropy, Δn_(F), of the optical retardation compensator plate withthe wavelength of light are set within such a range that viewing angledependency does not cause coloration on the liquid crystal screen. Thiscan further restrain coloration on the screen. The contrast variationsand coloration are also restrained better than only by the compensationfunction by the optical retardation compensator plate.

Moreover, as described above, in the first, second, and thirdarrangements, the liquid crystal display device is preferably arrangedso that the refractive index anisotropy, Δn_(L)(550), of the liquidcrystal material for the liquid crystal layer to light having awavelength of 550 nm is set within a range larger than 0.060 and smallerthan 0.120.

The setting can eliminate the phase difference that occurs to the liquidcrystal display element in accordance with the viewing angle. That canfurther restrain the contrast variations and tone reversion phenomenonin the right- and left-hand directions, as well as the colorationphenomenon that occurs depending upon the viewing angle.

In such an event the phase difference that occurs to the liquid crystaldisplay element in accordance with the viewing angle can be moreeffectively eliminated by setting the refractive index anisotropy,Δn_(L)(550), of the liquid crystal material for the liquid crystal layerto light having a wavelength of 550 nm so as to be within a range notsmaller than 0.070 and not larger than 0.095. This can surely restrainthe contrast variations and tone reversion phenomenon in the right- andleft-hand directions of the images displayed by the liquid crystaldisplay device.

Moreover, in the first, second, and third arrangements, the liquidcrystal display device is preferably arranged so that the or eachoptical retardation compensator plate is represented by a refractiveindex ellipsoid inclining by an inclination angle θ set within a rangeof 15° to 75°.

By setting the inclination angle of the refractive index ellipsoid to bewithin a range of 15° to 75° with respect to the or each opticalretardation compensator plate incorporated in the liquid crystal displaydevice, it is assured that the present invention provides theaforementioned compensation function for the phase difference by theoptical retardation compensator plate.

Moreover, in the first, second, and third arrangements, the liquidcrystal display device is preferably arranged so that the or eachoptical retardation compensator plate has a product, (n_(a)−n_(o))×d, ofthe difference between the principal refractive indices, na and nb, andthe thickness, d, of the optical retardation compensator plate, theproduct being set to be from 80 nm to 250 nm.

By setting the product, (n_(a)−n_(b))×d, of the difference between theprincipal refractive indices, na and nb, and the thickness, d, of theoptical retardation compensator plate, so as to be from 80 nm to 250 nmwith respect to the or each optical retardation compensator plateincorporated in the liquid crystal display device, it is assured thatthe present invention provides the aforementioned compensation functionfor the phase difference by the optical retardation compensator plate.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, are not in any way intendedto limit the scope of the claims of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the arrangement of a liquidcrystal display device in accordance with the first embodiment in adecomposed manner.

FIG. 2 is an explanatory drawing showing the relation between therubbing direction of the orientation film and the standard viewingdirection in the liquid crystal display device.

FIG. 3 is a perspective view illustrating the principle refractiveindices of an optical retardation compensator plate of the liquidcrystal display device.

FIG. 4 is a perspective view showing the optical arrangement of apolarizer plate and the optical retardation compensator plate of theliquid crystal display device in a decomposed manner.

FIG. 5 is an explanatory drawing showing the pretilt angle formed by thelinger axes of the liquid crystal molecules and the orientation film.

FIG. 6 is a perspective view showing a system for measuring the viewingangle dependency of the liquid crystal display device.

FIGS. 7(a), 7(b), and 7(c) are graphs showing the transmittance versusliquid crystal applied voltage characteristics of the liquid crystaldisplay devices of the first example and a comparative example for thefirst example.

FIGS. 8(a), 8(b), and 8(c) are graphs showing the transmittance versusliquid crystal applied voltage characteristics of the liquid crystaldisplay devices of the second example.

FIGS. 9(a), 9(b), and 9(c) are graphs showing the transmittance versusliquid crystal applied voltage characteristics of the liquid crystaldisplay devices of a comparative example for the second example.

FIG. 10 is a graph showing Δn(λ)/Δn_(L)(550), for wavelengths λ oflight, of an optical retardation compensator plate and a liquid crystalmaterial used as the liquid crystal layer of the liquid crystal displaydevice.

FIG. 11 is a schematic view showing the twisted orientation of liquidcrystal molecules in an TN liquid crystal display element.

DESCRIPTION OF THE EMBODIMENTS

[First Embodiment]

The following description will discuss the first embodiment inaccordance with the present invention.

As illustrated in FIG. 1, the liquid crystal display device of thepresent embodiment is provided with a liquid crystal display element 1,a pair of optical retardation compensator plates 2 and 3, and a pair ofpolarizer plates (polarizers) 4 and 5.

The liquid crystal display element 1 is constituted by electrodesubstrates 6 and 7 that are placed face to face with each other and theliquid crystal layer 8 that is sandwiched therebetween. The electrodesubstrate 6 is constructed as follows: a glass substrate (a transparentsubstrate) 9 is provided as a base, a transparent electrode 10, made ofITO (Indium Tin Oxide), is formed on the surface of the glass substrate9 facing the liquid crystal layer 8, and an orientation film 11 isformed on the transparent electrode 10. The electrode substrate 7 isconstructed as follows: a glass substrate (transparent substrate) 12 isprovided as a base, a transparent electrode 13, made of ITO, is formedon the surface of the glass substrate 12 facing the liquid crystal layer8, and an orientation film 14 is formed on the transparent electrode 13.

Although FIG. 1 shows a construction corresponding to two pixels forconvenience of explanation, the transparent electrodes 10 and 13, whichare strips with a predetermined width, are respectively placed on theglass substrates 9 and 12 with predetermined intervals all over theliquid crystal display element 1, and are designed so that they areorthogonal to each other on the glass substrates 9 and 12, when viewedin a direction perpendicular to the substrate surfaces. Portions atwhich the transparent electrodes 10 and 13 intersect each othercorrespond to pixels for carrying out display, and the pixels are placedin a matrix format over the entire structure of the present liquidcrystal display device.

The electrode substrates 6 and 7 are bonded by seal resin 15, and aliquid crystal layer 8 is sealed inside the space formed by the sealresin 15 and the electrode substrates 6 and 7. A voltage is applied viathe transparent electrodes 10 and 13 by a driving circuit (voltageapplication means) 17 according to display data.

The pretilt angle of the liquid crystal layer 8 of the present liquidcrystal display device is set so as to produce the best properties whencombined with the compensation function for phase difference by theoptical retardation compensator plates 2 and 3 (will be described laterin detail).

In the present liquid crystal display device, a unit, formed byincorporating optical retardation compensator plates 2 and 3 andpolarizer plates 4 and 5 into the above-mentioned liquid crystal displayelement 1, is referred to as a liquid crystal cell 16.

The orientation films 11 and 14 are treated with a rubbing technique inadvance so that the liquid crystal molecules between them are orientatedwith a twist angle of about 90°. As shown in FIG. 2, the rubbingdirection R₁ of the orientation film 11 and the rubbing direction R₂ ofthe orientation film 14 are set to be orthogonal to each other.

The optical retardation compensator plates 2 and 3 are provided betweenthe liquid crystal display element 1 and the respective polarizer plates4 and 5 disposed to flank the liquid crystal display element 1. Theoptical retardation compensator plates 2 and 3 are constituted by asupport base made of a transparent organic polymer and discotic liquidcrystal. The discotic liquid crystal is treated with an obliqueorientation technique or hybrid orientation, and crosslinked. As aresult, the optical retardation compensator plates 2 and 3 are formed soas to have a refractive index ellipsoid (will be described later indetail) that inclines to the optical retardation compensator plates 2and 3.

As for the support base of the optical retardation compensator plates 2and 3, triacetylcellulose (TAC), which is generally used for polarizerplates, is suitably applied with high reliability. Besides this,colorless, transparent organic polymeric films made of polycarbonate(PC), polyethyleneterephthalate (PET), etc., which are superior inenvironment resistance and chemical resistance, are also suitablyapplied.

As illustrated in FIG. 3, each of the optical retardation compensatorplates 2 and 3 has principal refractive indices na, nb and nc pointingin three different directions. The direction of the principal refractiveindex na coincides with the direction of the y-coordinate axis among themutually orthogonal x-, y-, and z-coordinate axes. The direction of theprincipal refractive index nb inclines by θ in the direction of arrow Awith respect to the z-coordinate axis (parallel to a normal to thesurface) that is perpendicular to the surface of the optical retardationcompensator plates 2 and 3, which surface corresponds to the screen.

The principal refractive indices na, nb, and nc of the opticalretardation compensator plates 2 and 3 are related to each other by theinequality: na=nc>nb. Therefore, there exists only one optic axis, andthe optical retardation compensator plates 2 and 3 have uniaxiality anda negative refractive index anisotropy. The first retardation value,(nc−na)×d, of the optical retardation compensator plates 2 and 3 equalsalmost 0 nm, since na=nc, while the second retardation value, (nc−nb)×d,is set to an arbitral value in a range from 80 nm to 250 nm. By settingthe second retardation value in such a range, the compensation functionfor phase difference by the optical retardation compensator plates 2 and3 is surely achieved. Note that (nc−na) and (nc−nb) each represent arefractive index anisotropy Δn_(F), and that d represents the thicknessof the optical retardation compensator plates 2 and 3.

The angle θ by which the direction of the principal refractive indicesnb of the optical retardation compensator plates 2 and 3 incline, i.e.the inclination angle θ of the refractive index ellipsoids, is set to anarbitrary value in the range 15°≦θ≦75°. By setting the inclination angleθ to such a value, regardless of whether the refractive index ellipsoidsincline clockwise or counterclockwise, the compensation function forphase difference by the optical retardation compensator plates 2 and 3is surely achieved.

Instead of using the two optical retardation compensator plates 2 and 3,only one of them may be used and disposed on one side. Alternatively,both the optical retardation compensator plates 2 and 3 can be disposedon one side, one of them overlapping the other. As a furtheralternative, three or more optical retardation compensator plates may beused.

As illustrated in FIG. 4, in the present liquid crystal display device,the polarizer plates 4 and 5 in the liquid crystal display element 1 arearranged so that their absorption axes AX₁ and AX₂ are parallel to therubbing directions R₁ and R₂ of the orientation films 11 and 14respectively (see FIG. 1). In the present liquid crystal display device,since the rubbing directions R₁ and R₂ are orthogonal to each other, theabsorption axes AX₁ and AX₂ are also orthogonal to each other.

Here, as illustrated in FIG. 3, the direction D is defined as adirection formed by projecting the direction of the principal refractiveindex nb, which inclines in such a direction to impart anisotropy to theoptical retardation compensator plates 2 and 3, onto the surface of theoptical retardation compensator plates 2 and 3. As illustrated in FIG.4, the optical retardation compensator plate 2 is placed so that thedirection D (direction D₁) is parallel to the rubbing direction R₁, andthe optical retardation compensator plate 3 is placed so that thedirection D (direction D₂) is parallel to the rubbing direction R₂.

With the above-mentioned arrangement of the optical retardationcompensator plates 2 and 3 and the polarizer plates 4 and 5, the presentliquid crystal display device can carry out so-called Normally Whitedisplay wherein rays of light are allowed to pass during OFF time sothat white display is provided.

In general, in optical anisotropic materials such as liquid crystal andoptical retardation compensator plates (phase difference films), theabove-mentioned anisotropy of the three-dimensional principal refractiveindices na, nc and nb is represented by a refractive index ellipsoid.The refractive-index anisotropy Δn assumes different values depending onwhich direction the refractive index ellipsoid is observed.

Next, the aforementioned setting of the pretilt angle for the liquidcrystal layer 8 will be explained in detail.

As illustrated in FIG. 5, the pretilt angle is the angle φ formed by theorientation film 14 (11) and the longer axes of liquid crystal molecules20, and determined by the combination of rubbing treatment of the liquidcrystal material and the rubbing of the orientation films 11 and 14.

As mentioned earlier, the pretilt angle of the liquid crystal layer 8 ofthe present liquid crystal display device is set to produce the bestproperties when combined with the compensation function for phasedifference by the optical retardation compensator plates 2 and 3.Specifically, the pretilt angle is set in a range that does not causetone reversion in the opposite viewing direction in a halftone displaystate where a voltage that is close to the threshold voltage for theliquid crystal is applied to the liquid crystal. Here, since theNormally White display mode is selected, the halftone display state isclose to white color. Hereinafter, the halftone display state close towhite color will be referred to as white tone.

It has been confirmed through experiments that the larger the pretiltangles are, the less likely the tone reversion occurs in the oppositeviewing direction, whereas too large pretilt angles cause an abruptdecrease in luminance in the standard viewing direction when white toneis being displayed. Thus, the pretilt angle also needs to be set withinsuch a range that luminance does not decrease abruptly in the standardviewing direction when white tone is being displayed.

More specifically, used as the orientation films 11 and 14 and theliquid crystal material is a combination of orientation films and aliquid crystal material that results in a pretilt angle more than 2° andless than 12°. More preferable is a combination that results in apretilt angle not less than 4° and not more than 10°.

The setting of the pretilt angle in a range more than 2° and less than12° enables the liquid crystal display device to be free fromproblem-posing tone reversion in the opposite viewing direction whenwhite tone is being displayed and to be viewed in every direction at theviewing angle of 50° which is typically required for liquid crystaldisplay devices.

Especially, the setting of the pretilt angle in a range not less than 4°and not more than 10° enables the liquid crystal display device to beviewed without tone reversion at all in the opposite viewing directionat the viewing angle of 70° when white tone is being displayed.

Selected as the liquid crystal material for the liquid crystal layer 8of the liquid crystal display device in accordance with the presentinvention is a liquid crystal material of which the refractive indexanisotropy, Δn_(L)(550), to light having a wavelength of 550 nm isdesigned to be within a range larger than 0.060 and smaller than 0.120.More preferably, a liquid crystal material of which the refractive indexanisotropy, Δn_(L)(550), is designed to be within a range not smallerthan 0.070 and not larger than 0.095 is used.

As a result, the optical retardation compensator plates 2 and 3 becomecapable of compensating for the phase difference. And so does thesetting of the pretilt angle in the range above. Moreover, the decreasein contrast ratio in the opposite viewing direction can be furtherrestrained, and the tone reversion phenomenon in the right- andleft-hand directions can be further restrained.

As explained so far, the liquid crystal display device of the presentembodiment includes, between the liquid crystal display element 1 andthe polarizer plates 4 and 5, the optical retardation compensator plates2 and 3 each represented by a refractive index ellipsoid having threeprincipal refractive indices, na, nb, and nc, mutually related by theinequality na=nc>nb, the refractive index ellipsoid inclining as thedirection of the principal refractive index nb parallel to the normal tothe surface and the direction of either the principal refractive indexna or nc in the surface recline either clockwise or counterclockwisearound the direction of the principal refractive index nc or na in thesurface,

wherein the pretilt angle of the liquid crystal layer 8 is set withinsuch a range that tone reversion does not occur in the opposite viewingdirection when halftone is being displayed by applying to the liquidcrystal a voltage that is close to the threshold voltage for the liquidcrystal.

As a result, the tone reversion phenomenon that occurs in the oppositedirection according to the viewing angle when white tone (becauseNormally White display is being adopted) is being displayed can be,above all, effectively restrained by the compensation function for phasedifference that occurs to the liquid crystal display element 1 accordingto the viewing angle by the setting of the pretilt angle in the rangeabove, as well as by the compensation function by the opticalretardation compensator plates 2 and 3. Besides, the contrast variationscan be improved, resulting in display of high quality images.

Besides, the liquid crystal display device of the present embodimentemploys as the liquid crystal material for the liquid crystal layer 8 aliquid crystal material of which the refractive index anisotropy,Δn_(L)(550), to light having a wavelength of 550 nm, is designed to bewithin a range larger than 0.060 and smaller than 0.120. Therefore, theoptical retardation compensator plates 2 and 3 become capable ofcompensating for the phase difference. And so does the setting of thepretilt angle in the range above. Moreover, the decrease in contrastratio in the opposite viewing direction can be further restrained, andthe tone reversion phenomenon in the right- and left-hand directions canbe further restrained.

Note that although the liquid crystal display device of Normally Whitedisplay has been taken as an example in the description above, the sameeffects can be obtained with a liquid crystal display device of NormallyBlack display by achieving compensation function for phase difference bythe setting of the pretilt angle within such a range that tone reversiondoes not occur in the opposite viewing direction when halftone (blacktone) is being displayed by applying to the liquid crystal a voltagethat is close to the threshold voltage for the liquid crystal, as wellas by the compensation function by the optical retardation compensatorplates 2 and 3.

Note also that although the liquid crystal display device of a simplematrix method has been taken as an example in the description of theembodiment above, the present invention can be applied to a liquidcrystal display device of an active matrix method using active switchingelements such as TFTs.

[Second Embodiment]

The following description will discuss another embodiment in accordancewith the present invention. Here, for convenience, members of thepresent embodiment that have the same function as members of the firstembodiment, and that are mentioned in the first embodiment are indicatedby the same reference numerals and description thereof is omitted.

The liquid crystal display device of the present embodiment isconfigured almost in the same manner as is the liquid crystal displaydevice of the first embodiment shown in FIG. 1, except the followingpoints:

The liquid crystal display device of the first embodiment includes theliquid crystal layer 8 of which the pretilt angle is set in a range thatdoes not cause tone reversion in the opposite viewing direction in ahalftone display state where a voltage that is close to the thresholdvoltage for the liquid crystal is applied to the liquid crystal layer 8,so as to produce the best properties when combined with the compensationfunction for phase difference by the optical retardation compensatorplates 2 and 3.

The liquid crystal display device of the present embodiment, bycontrast, includes a liquid crystal layer 8 such that the value of theapplied voltage for displaying halftone obtained by applying to theliquid crystal layer 8 a voltage that is close to the threshold voltagefor the liquid crystal is set within such a range that tone reversiondoes not occur in the opposite viewing direction when halftone is beingdisplayed, so as to produce the best properties when combined with thecompensation function for phase difference by the optical retardationcompensator plates 2 and 3.

Next, the above differences will be explained in detail.

Since the liquid crystal display device of the present embodiment is ofNormally White display, the value of the applied voltage for realizinghalftone display state where a voltage that is close to the thresholdvoltage for the liquid crystal is applied to the liquid crystal, i.e.white tone, is set within such a range that tone reversion does notoccur in the opposite viewing direction when that voltage is beingapplied.

It has been confirmed through experiments that the lower thetransmittance when white tone is being displayed is, the less likely thetone reversion occurs in the opposite viewing direction when white toneis being displayed. On the other hand, too low transmittances cause anabrupt decrease in luminance in the standard viewing direction and inthe right- and left-hand directions. Thus, the voltage applied to theliquid crystal that determines the transmittance when white tone isbeing displayed needs to be set also within such a range that luminancedoes not decrease abruptly in the standard viewing direction and in theright- and left-hand directions when white tone is being displayed.

Specifically, the voltage applied to the liquid crystal when white toneis being displayed is set so that the transmittance when white tone isbeing displayed is higher than 85% that in the OFF state. In such acase, the voltage applied to the liquid crystal when white tone is beingdisplayed is more preferably set so that the transmittance when whitetone is being displayed is in a range not less than 90% and not morethan 97% that in the OFF state. The OFF state refers to a state wherethe voltage applied to the liquid crystal is zero.

The setting of the voltage applied to the liquid crystal when white toneis being displayed so that the transmittance when white tone is beingdisplayed is higher than 85% that in the OFF state enables the liquidcrystal display device to be free from problem-posing tone reversion inthe opposite viewing direction when white tone is being displayed and tobe viewed in every direction at the viewing angle of 50° which istypically required for liquid crystal display devices.

Especially, the setting of the voltage applied to the liquid crystalwhen white tone is being displayed so that the transmittance when whitetone is being displayed is in a range not less than 90% and not morethan 97% that in the OFF state enables the liquid crystal display deviceto be viewed without tone reversion at all in the opposite viewingdirection at the viewing angle of 70° when white tone is beingdisplayed.

As explained above, the liquid crystal display device of the presentembodiment includes, between the liquid crystal display element 1 andthe polarizer plates 4 and 5, the optical retardation compensator plates2 and 3 each represented by a refractive index ellipsoid having threeprincipal refractive indices, na, nb, and nc, mutually related by theinequality na=nc>nb, the refractive index ellipsoid inclining as thedirection of the principal refractive index nb parallel to the normal tothe surface and the direction of either the principal refractive indexna or nc in the surface recline either clockwise or counterclockwisearound the direction of the principal refractive index nc or na in thesurface,

wherein the value of the applied voltage for realizing halftone displaywhere a voltage that is close to the threshold voltage for the liquidcrystal is applied to the liquid crystal is set within such a range thattone reversion does not occur in the opposite viewing direction in thestate where that voltage is applied.

As a result, the tone reversion phenomenon that occurs in the oppositedirection according to the viewing angle when white tone (becauseNormally White display is being adopted) is being displayed can be,above all, effectively restrained by the compensation function for phasedifference that occurs to the liquid crystal display element 1 accordingto the viewing angle by the setting of the voltage applied to the liquidcrystal when white tone is being displayed in the range above, as wellas by the compensation function by the optical retardation compensatorplates 2 and 3. Besides, the contrast variations can be improved,resulting in display of high quality images.

Besides, similarly to the liquid crystal display device of the previousembodiment, by employing as the liquid crystal material for the liquidcrystal layer 8 a liquid crystal material of which the refractive indexanisotropy, Δn_(L)(550), to light having a wavelength of 550 nm isdesigned to be within a range larger than 0.060 and smaller than 0.120,and more preferably, within a range not smaller than 0.070 and notlarger than 0.095, the decrease in contrast ratio in the oppositeviewing direction and the tone reversion phenomenon in the right- andleft-hand directions can be further restrained by the compensationfunction for phase difference by the setting of the voltage applied tothe liquid crystal when white tone is being displayed in the rangeabove, as well as by the compensation function by the opticalretardation compensator plates 2 and 3.

Note that although the liquid crystal display device of Normally Whitedisplay has been taken as an example in the description above, the sameeffects can be obtained with a liquid crystal display device of NormallyBlack display by achieving compensation function for phase difference bythe setting of the voltage to be applied to the liquid crystal forhalftone (black tone) display obtained by applying to the liquid crystala voltage that is close to the threshold voltage for the liquid crystalwithin such a range that tone reversion does not occur in the oppositeviewing direction when halftone is being displayed, as well as by thecompensation function by the optical retardation compensator plates 2and 3.

Note also that similarly to the first embodiment, apart from the liquidcrystal display device of a simple matrix method, the present inventioncan be applied to a liquid crystal display device of an active matrixmethod using active switching elements such as TFTs.

[Third Embodiment]

The following description will discuss another embodiment in accordancewith the present invention. Here, for convenience, members of thepresent embodiment that have the same function as members of theprevious embodiments, and that are mentioned in the previous embodimentsare indicated by the same reference numerals and description thereof isomitted.

The liquid crystal display device of the present embodiment isconfigured almost in the same manner as is the liquid crystal displaydevice of the first embodiment shown in FIG. 1, except the followingpoints:

The liquid crystal display device of the first embodiment includes theliquid crystal layer 8 of which the pretilt angle is set in a range thatdoes not cause tone reversion in the opposite viewing direction in ahalftone display state where a voltage that is close to the thresholdvoltage for the liquid crystal is applied to the liquid crystal layer 8,so as to produce the best properties when combined with the compensationfunction for phase difference by the optical retardation compensatorplates 2 and 3.

The liquid crystal display device of the present embodiment, bycontrast, includes a liquid crystal layer 8 such that the ratios of thevariation in the refractive index anisotropy, Δn_(L), of the liquidcrystal material for the liquid crystal layer 8 with the wavelength oflight and of the variation in the refractive index anisotropy, Δn_(F),of the optical retardation compensator plate with the wavelength oflight are set within such a range that viewing angle dependency does notcause coloration on the liquid crystal screen, so as to produce the bestproperties when combined with the compensation function for phasedifference by the optical retardation compensator plates 2 and 3.

Next, the above differences will be explained in detail.

Setting the ratios of the variation in the refractive index anisotropy,Δn_(L), of the liquid crystal material for the liquid crystal layer 8with the wavelength of light and of the variation in the refractiveindex anisotropy, Δn_(F), of the optical retardation compensator platewith the wavelength of light within such a range that viewing angledependency does not cause coloration on the liquid crystal screen refersto, in more specific terms, a combined use of optical retardationcompensator plates 2 and 3 with a liquid crystal material that satisfyat least one of the range-setting conditions {circumflex over (1)} and{circumflex over (2)} below:

{circumflex over (1)} The ratio, Δn_(L)(450)/Δn_(L)(550), of therefractive index anisotropy, Δn_(L)(450), of the liquid crystal materialfor the liquid crystal layer 8 to light having a wavelength of 450 nmand the refractive index anisotropy, Δn_(L)(550), thereof to lighthaving a wavelength of 550 nm, and the ratio, Δn_(F)(450)/Δn_(F)(550),of the refractive index anisotropy, Δn_(F)(450), of the opticalretardation compensator plates 2 and 3 to light having a wavelength of450 nm and the refractive index anisotropy, Δn_(F)(550), thereof tolight having a wavelength of 550 nm are set to satisfy the inequality:$0 \leq \frac{\left( {\Delta\quad{{n_{L}(450)}/\Delta}\quad{n_{L}(550)}} \right) - 1}{\left( {\Delta\quad{{n_{F}(450)}/\Delta}\quad{n_{F}(550)}} \right) - 1} < 0.35$

and are, more preferably, set to satisfy the inequality:$0 \leq \frac{\left( {\Delta\quad{{n_{L}(450)}/\Delta}\quad{n_{L}(550)}} \right) - 1}{\left( {\Delta\quad{{n_{F}(450)}/\Delta}\quad{n_{F}(550)}} \right) - 1} \leq 0.25$

{circumflex over (2)} The ratio, Δn_(L)(650)/Δn_(L)(550), of therefractive index anisotropy, Δn_(L)(650), of the liquid crystal materialfor the liquid crystal layer 8 to light having a wavelength of 650 nmand the refractive index anisotropy, Δn_(L)(550), thereof to lighthaving a wavelength of 550 nm, and the ratio, Δn_(F)(650)/Δn_(F)(550),of the refractive index anisotropy, Δn_(F)(650), of the opticalretardation compensator plates 2 and 3 to light having a wavelength of650 nm and the refractive index anisotropy, Δn_(F)(550), thereof tolight having a wavelength of 550 nm are set to satisfy the inequality:$0 \leq \frac{1 - \left( {\Delta\quad{{n_{L}(650)}/\Delta}\quad{n_{L}(550)}} \right)}{1 - \left( {\Delta\quad{{n_{F}(650)}/\Delta}\quad{n_{F}(550)}} \right)} < 0.27$

and are, more preferably, set to satisfy the inequality:$0 \leq \frac{1 - \left( {\Delta\quad{{n_{L}(650)}/\Delta}\quad{n_{L}(550)}} \right)}{1 - \left( {\Delta\quad{{n_{F}(650)}/\Delta}\quad{n_{F}(550)}} \right)} \leq 0.20$

The use of a liquid crystal material and optical retardation compensatorplates designed to satisfy at least one of the conditions {circumflexover (1)} and {circumflex over (2)} permits the effective restraint in,above all, the coloration phenomenon on the display screen, in additionto the restraint in the contrast variations, tone reversion phenomenon,and coloration phenomenon caused by the viewing angle dependency of thedisplay screen by the compensation function for phase difference by theoptical retardation compensator plates 2 and 3.

To be more specific, although in some instances still incapable ofcompletely eliminating coloration at the viewing angle of 50°, which isthe viewing angle typically required for liquid crystal display devices,satisfying at least one of the wider conditions of {circumflex over (1)}and {circumflex over (2)} enables the liquid crystal display device tobe viewed in every direction without problems for real use.

And, satisfying at least one of the preferred conditions of {circumflexover (1)} and {circumflex over (2)} enables the liquid crystal displaydevice to be viewed in every direction without any coloration at all atthe viewing angle of 70°.

Furthermore, satisfying at least one of {circumflex over (1)} and{circumflex over (2)} restrains also contrast variations and tonereversion phenomenon better than does the compensation function by theoptical retardation compensator plates 2 and 3 alone.

FIG. 10 shows Δn(λ)/Δn(550) for wavelengths λ with a combination of aliquid crystal material that can be used as the liquid crystal layer 8of the present liquid crystal display device and of optical retardationcompensator plates that can be used as the optical retardationcompensator plates 2 and 3. The solid curved line a showsΔn_(L)(λ)/Δn_(L)(550) for wavelengths λ of a liquid crystal material,while the alternative long and short dash line b showsΔn_(F)(λ)/Δn_(F)(550) for wavelengths λ of an optical retardationcompensator plate.

As explained above, the liquid crystal display device of the presentembodiment includes, between the liquid crystal display element 1 andthe polarizer plates 4 and 5, the optical retardation compensator plates2 and 3 each represented by a refractive index ellipsoid having threeprincipal refractive indices, na, nb, and nc, mutually related by theinequality na=nc>nb, the refractive index ellipsoid inclining as thedirection of the principal refractive index nb parallel to the normal tothe surface and the direction of either the principal refractive indexna or nc in the surface recline either clockwise or counterclockwisearound the direction of the principal refractive index nc or na in thesurface,

wherein the ratios of the variation in the refractive index anisotropy,Δn_(L), of the liquid crystal material for the liquid crystal layer 8with the wavelength of light and of the variation in the refractiveindex anisotropy, Δn_(F), of the optical retardation compensator platewith the wavelength of light are set within such a range that viewingangle dependency does not cause coloration on the liquid crystal screen.

As a result, the coloration phenomenon caused by the viewing angledependency of the display screen can be, above all, effectivelyrestrained by the compensation function for phase difference that occursto the liquid crystal display element 1 according to the viewing angleby the setting of the ratios of the variation in the refractive indexanisotropy, Δn_(L), of the liquid crystal material for the liquidcrystal layer 8 with the wavelength of light and of the variation in therefractive index anisotropy, Δn_(F), of the optical retardationcompensator plate with the wavelength of light in the range above, aswell as by the compensation function by the optical retardationcompensator plates 2 and 3. Besides, the contrast variations and tonereversion phenomenon can be improved, resulting in display of highquality images.

Besides, similarly to the liquid crystal display device of the previousembodiments, by employing as the liquid crystal material for the liquidcrystal layer 8 a liquid crystal material of which the refractive indexanisotropy, Δn_(L)(550) to light having a wavelength of 550 nm, isdesigned to be within a range larger than 0.060 and smaller than 0.120,and more preferably, within a range not smaller than 0.070 and notlarger than 0.095, the decrease in contrast ratio in the oppositeviewing direction and the tone reversion phenomenon in the right- andleft-hand directions can be further restrained by the compensationfunction for phase difference by the setting of the ratios of thevariations in the range above, as well as by the compensation functionby the optical retardation compensator plates 2 and 3.

Note that although the liquid crystal display device of Normally Whitedisplay has been taken as an example in the description above, the sameeffects can be obtained with a liquid crystal display device of NormallyBlack display.

Note also that similarly to the first embodiment, apart from the liquidcrystal display device of a simple matrix method, the present inventioncan be applied to a liquid crystal display device of an active matrixmethod using active switching elements such as TFTs.

The following description will explain examples that substantiate theeffects of the liquid crystal display devices of the first, second, andthird embodiments.

FIRST EXAMPLE

The present example is to substantiate the effects of the liquid crystaldisplay devices of the first and second embodiments. Here, seven samplecells #1 to #7 were prepared by using Optomer AL (product name),available from Japan Synthetic Rubber Co., Ltd., as the orientationfilms 11 and 14 of the liquid crystal cell 16 of the liquid crystaldisplay device shown in FIG. 1, selecting suitable liquid crystalmaterials to set the pretilt angles to 2.0°, 3.0°, 4.0°, 5.0°, 10.0°,11.0°, and 12.0° with respect to the orientation films 11 and 14, andsetting the thickness of the cells of the liquid crystal layers 8 to 5μm.

Homogeneous cells were prepared by injecting thereinto the materials forthe sample cells #1 to #7, and measured with a pretilt angle measuringdevice, NSMAP-3000LCD (Sigma Optical Machinery Co., Ltd.), for thepretilt angles of the sample cells #1 to #7.

Used as the optical retardation compensator plates 2 and 3 of the samplecells #1 to #7 are those constituted by a transparent support base(e.g., triacetylcellulose (TAC)) on which discotic liquid crystal isapplied. The discotic liquid crystal is treated with an obliqueorientation technique, and crosslinked. The optical retardationcompensator plates 2 and 3 each have resulting first and secondretardation values of 0 and 100 nm respectively, a principal refractiveindex nb inclining by 20° in the direction of arrow A with respect tothe z-coordinate axis of the x-, y-, and z-coordinates system, and aprincipal refractive index nc inclining by 20° in the direction of arrowB with respect to the x-coordinate axis (that is, the inclination angleof the refractive index ellipsoid θ=20°).

Tables 1 to 7 show results of visual observations of the sample cells #1to #7 under white light with various voltages applied for white tone.

TABLE 1 Applied voltage for white tone set to derive a transmittance100% that in the OFF state Viewing Pretilt angle (°) Angle 2.0 3.0 4.05.0 10.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° x₁ ∘ ∘ ∘ ∘ ∘ x₂ 60° x₁ ∘∘ ∘ ∘ ∘ x₂ 70° x₁ Δ₂ Δ₁ ∘ ∘ x₂ x₂ (“∘” represents that no tone reversionwas observed in the opposite viewing direction, “Δ₁” represents that notone reversion was observed in the opposite viewing direction, but thattone is distorted within the extent that did not pose any problem forreal use, “Δ₂” represents than tone reversion was observed in theopposite viewing direction within the extent that did not pose anyproblem for real use, “x₁” represents that tone reversion was observedin the opposite viewing direction, and “x₂” represents that a decreasein luminance was evident in the standard viewing direction to the extentunbearable for real use.)

Table 1 shows, supposing that the transmittance along the normal to thesurface of the liquid crystal cell 16 as 100% in an OFF state where thevoltage applied to the liquid crystal layer is zero, results of displayconditions when white tone is being displayed by setting a value thatderives 100% of the transmittance along the normal for each sample cell.

Table 1 shows that in a case where the voltage when white tone was beingdisplayed was set to cause the ratio of the transmittance when whitetone is being displayed to be 100%, the sample cells #4 and #5, havingrespective pretilt angles of 5.0° and 10.0°, displayed high qualityimages with no tone reversion being observed in the opposite viewingdirection at a viewing angle of 70°.

Up to a viewing angle of 60°, the sample cells #2 and #3, havingrespective pretilt angles of 3.0° and 4.0°, displayed high qualityimages with no tone reversion being observed in the opposite viewingdirection. At a viewing angle of 70°, tone reversion was observed withthe sample cell #2 within the extent that did not pose any problem forreal use, and tone was distorted, although not reversed, with the samplecell #3. The sample cells #3 and #4 however did not pose any problem forreal use at the viewing angle of 70°.

Up to a viewing angle of 60°, the sample cell #6 with the pretilt angleof 11.0° displayed high quality images. However, at a viewing angle of70°, a decrease in luminance was evident in the standard viewingdirection to the extent unbearable for real use.

With the sample cell #1, having a pretilt angle of 2.0°, tone reversionwas observed in the opposite viewing direction at a viewing angle as lowas 50°. With the sample cell #7, having a pretilt angle of 12.0°, adecrease in luminance was evident in the standard viewing direction at aviewing angle as low as 50° to the extent unbearable for real use.

TABLE 2 Applied voltage for white tone set to derive a transmittance 97%that in the OFF state Viewing Pretilt angle (°) Angle 2.0 3.0 4.0 5.010.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° x₁ ∘ ∘ ∘ ∘ ∘ x₂ 60° x₁ Δ₁ ∘∘ ∘ x₂ x₂ 70° x₁ x₁ ∘ ∘ ∘ x₂ x₂ (“∘” represents that no tone reversionwas observed in the opposite viewing direction, “Δ₁” represents that notone reversion was observed in the opposite viewing direction, but thattone is distorted within the extent that did not pose any problem forreal use, “x₁” represents that tone reversion was observed in theopposite viewing direction, and “x₂” represents that a decrease inluminance was evident in the standard viewing direction to the extentunbearable for real use.)

Table 2 shows results observed by setting a voltage for white tone foreach sample cell to cause the transmittance for white tone to be 97%that in an OFF state.

Table 2 shows that in a case where the voltage when white tone was beingdisplayed was set to cause the ratio of the transmittance when whitetone was being displayed to be 97%, the sample cells #3, #4, and #5,having respective pretilt angles of 4.0°, 5.0°, and 10.0°, displayedhigh quality images with no tone reversion being observed in theopposite viewing direction at a viewing angle of 70°.

Up to a viewing angle of 50°, the sample cell #2, having a pretilt angleof 3.0°, displayed high quality images with no tone reversion beingobserved in the opposite viewing direction. At a viewing angle of 60°,tone was distorted with the sample cell #2. However, the sample cell #2did not pose any problem for real use, because tone was not reversed.The sample cell #6 with the pretilt angle of 11.0° displayed highquality images up to a viewing angle of 50°. However, at a viewing angleof 60°, a decrease in luminance was evident in the standard viewingdirection to the extent unbearable for real use.

With the sample cell #1, having a pretilt angle of 2.0°, tone reversionwas observed in the opposite viewing direction at a viewing angle as lowas 50°. With the sample cell #7, having a pretilt angle of 12.0°, adecrease in luminance was evident in the standard viewing direction at aviewing angle as low as 50° to the extent unbearable for real use.

TABLE 3 Applied voltage for white tone set to derive a transmittance 95%that in the OFF state Viewing Pretilt angle (°) Angle 2.0 3.0 4.0 5.010.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° x₁ ∘ ∘ ∘ ∘ ∘ x₂ 60° x₁ Δ₁ ∘∘ ∘ x₂ x₂ 70° x₁ x₁ ∘ ∘ ∘ x₂ x₂ (“∘” represents that no tone reversionwas observed in the opposite viewing direction, “Δ₁” represents that notone reversion was observed in the opposite viewing direction, but thattone is distorted within the extent that did not pose any problem forreal use, “x₁” represents that tone reversion was observed in theopposite viewing direction, and “x₂” represents that a decrease inluminance was evident in the standard viewing direction to the extentunbearable for real use.)

Table 3 shows results observed by setting a voltage for white tone foreach sample cell to cause the transmittance to be 95% that in an OFFstate. Those results were the same as in Table 2 in which the voltagewas set to cause the transmittance for white tone to be 97%.

TABLE 4 Applied voltage for white tone set to derive a transmittance 92%that in the OFF state Viewing Pretilt angle (°) Angle 2.0 3.0 4.0 5.010.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° Δ₂ ∘ ∘ ∘ ∘ ∘ x₂ 60° x₁ ∘ ∘ ∘∘ x₂ x₂ 70° x₁ Δ₂ ∘ ∘ ∘ x₂ x₂ (“∘” represents that no tone reversion wasobserved in the opposite viewing direction, “Δ₂” represents that tonereversion was observed in the opposite viewing direction within theextent that did not pose any problem for real use, “x₁” represents thattone reversion was observed in the opposite viewing direction, and “x₂”represents that a decrease in luminance was evident in the standardviewing direction to the extent unbearable for real use.)

Table 4 shows results observed by setting a voltage for white tone foreach sample cell to cause the ratio of the transmittance for white toneto be 92% that in an OFF state.

Table 4 shows that in a case where the voltage when white tone was beingdisplayed was set to cause the ratio of the transmittance for white toneto be 92%, the sample cells #3, #4, and #5, having respective pretiltangles of 4.0°, 5.0°, and 10.0°, displayed high quality images with notone reversion being observed in the opposite viewing direction at aviewing angle of 70°.

Up to a viewing angle of 60°, the sample cell #2, having a pretilt angleof 3.0°, displayed high quality images with no tone reversion beingobserved in the opposite viewing direction. At a viewing angle of 70°,tone was reversed with the sample cell #2. However, the tone reversionwas within the extent that did not pose any problem for real use. Thesample cell #6 with the pretilt angle of 11.0° displayed high qualityimages up to a viewing angle of 50°. However, at a viewing angle of 60°,a decrease in luminance was evident in the standard viewing direction tothe extent unbearable for real use. Tone reversion was observed at aviewing angle of 50° with the sample cell #1, having a pretilt angle of2.0°, within the extent that did not pose any problem for real use.

With the sample cell #7, having a pretilt angle of 12.0°, a decrease inluminance was evident in the standard viewing direction at a viewingangle as low as 50° to the extent unbearable for real use.

TABLE 5 Applied voltage for white tone set to derive a transmittance 90%that in the OFF state Viewing Pretilt angle (°) Angle 2.0 3.0 4.0 5.010.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° Δ₁ ∘ ∘ ∘ ∘ Δ₃ x₂ 60° Δ₂ ∘ ∘∘ Δ₃ x₂ x₂ 70° x₁ Δ₁ ∘ ∘ x₂ x₂ x₂ (“∘” represents that no tone reversionwas observed in the opposite viewing direction, “Δ₁” represents that notone reversion was observed in the opposite viewing direction, but thattone is distorted within the extent that did not pose any problem forreal use, “Δ₂” represents that tone reversion was observed in theopposite viewing direction within the extent that did not pose anyproblem for real use, “Δ₃” represents in luminance was observed in thestandard viewing direction within the extent that did not pose anyproblem for real use, “x₁” represents that tone reversion was observedin the opposite viewing direction, and “x₂” represents that a decreasein luminance was evident in the standard viewing direction to the extentunbearable for real use.)

Table 5 shows results observed by setting a voltage for white tone foreach sample cell to cause the ratio of the transmittance for white toneto be 90% that in an OFF state.

Table 5 shows that in a case where the voltage when white tone was beingdisplayed was set to cause the ratio of the transmittance when whitetone was being displayed to be 90%, the sample cells #3 and #4, havingrespective pretilt angles of 4.0° and 5.0°, displayed high qualityimages with no tone reversion being observed in the opposite viewingdirection at a viewing angle of 70°.

Up to a viewing angle of 50°, the sample cell #5, having a pretilt angleof 10.0°, displayed high quality images. At a viewing angle of 60°, adecrease in luminance was observed in the standard viewing directionwithin the extent that did not pose any problem for real use. Up to aviewing angle of 60°, the sample cell #2, having a pretilt angle of3.0°, displayed high quality images with no tone reversion beingobserved even in the opposite viewing direction. At a viewing angle of70°, tone was distorted within the extent that did not pose any problemfor real use, but no tone reversion was observed. With the sample cell#6 with the pretilt angle of 11.0°, a decrease in luminance was observedin the standard viewing direction at a viewing angle of 50° within theextent that did not pose any problem for real use. With the sample cell#1, having a pretilt angle of 2.0°, tone was distorted at a viewingangle of 50° and reversed at a viewing angle of 60° within the extentthat did not pose any problem for real use.

With the sample cell #7, having a pretilt angle of 12°, a decrease inluminance was evident in the standard viewing direction at a viewingangle as low as 50° to the extent unbearable for real use.

TABLE 6 Applied voltage for white tone set to derive a transmittance 87%that in the OFF state Viewing Pretilt angle (°) Angle 2.0 3.0 4.0 5.010.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° ∘ ∘ ∘ ∘ Δ₃ x₂ x₂ 60° Δ₃ Δ₃Δ₃ Δ₃ Δ₃ x₂ x₂ 70° x₂ x₂ x₂ x₂ x₂ x₂ x₂ (“∘” represents that no tonereversion was observed in the opposite viewing direction, “Δ₃”represents that a decrease in luminance was observed in the standardviewing direction within the extent that did not pose any problem forreal use, and “x₂” represents that a decrease in luminance was evidentin the standard viewing direction to the extent unbearable for realuse.)

Table 6 shows results observed by setting a voltage for white tone foreach sample cell to cause the ratio of the transmittance for white toneto be 87% that in an OFF state.

Table 6 shows that in a case where the voltage when white tone was beingdisplayed was set to cause the ratio of the transmittance when whitetone was being displayed to be 87%, the sample cells #2, #3, and #4,having respective pretilt angles of 3.0°, 4.0° and 5.0°, displayed highquality images up to a viewing angle of 50°. However, at a viewing angleof 60°, a decrease in luminance was observed in the standard viewingdirection within the extent that did not pose any problem for real use.At a viewing angle of 70°, a decrease in luminance was evident in thestandard viewing direction to the extent unbearable for real use.

At viewing angles of 50° and 60°, a decrease in luminance was observedwith the sample cell #5, having a pretilt angle of 10.0°, in thestandard viewing direction within the extent that did not pose anyproblem for real use. At a viewing angle of 70°, a decrease in luminancewas evident in the standard viewing direction to the extent unbearablefor real use.

With the sample cells #6 and #7, having respective pretilt angles of11.0° and 12.0°, a decrease in luminance was evident in the standardviewing direction at a viewing angle as low as 50° to the extentunbearable for real use.

Up to a viewing angle of 50°, the sample cell #1, having a pretilt angleof 2.0°, displayed high quality images with no tone reversion beingobserved in the opposite viewing direction. However, at a viewing angleof 60°, a decrease in luminance was observed in the standard viewingdirection within the extent that did not pose any problem for real use.At a viewing angle of 70°, a decrease in luminance was evident in thestandard viewing direction to the extent unbearable for real use.

TABLE 7 Applied voltage for white tone set to derive a transmittance 85%that in the OFF state Viewing Pretilt angle (°) Angle 2.0 3.0 4.0 5.010.0 11.0 12.0 (θ) #1 #2 #3 #4 #5 #6 #7 50° Δ₃ x₃ x₃ x₃ x₃ x₃ x₃ 60° x₃x₃ x₃ x₃ x₃ x₃ x₃ 70° x₃ x₃ x₃ x₃ x₃ x₃ x₃ (“Δ₃” represents that adecrease in luminance was observed in the standard viewing directionwithin the extent that did not pose any problem for real use, and “x₃”represents that a decrease in luminance was evident in the standardviewing direction and in the right- and left-hand directions to theextent unbearable for real use.)

Table 7 shows results observed by setting a voltage for white tone foreach sample cell to cause the ratio of the transmittance for white toneto be 85% that in an OFF state.

Table 7 shows that in a case where the voltage when white tone was beingdisplayed was set to cause the ratio of the transmittance when whitetone was being displayed to be 85%, a decrease in luminance was evidentwith the sample cells #2, #3, #4, #5, #6, and #7, having respectivepretilt angles of 3.0°, 4.0° 5.0°, 10.0°, 11.0° and 12.0°, in thestandard viewing direction and in the right- and left-hand directions ata viewing angle as low as 50° to the extent unbearable for real use.

At a viewing angle of 50°, a decrease in luminance was observed with thesample cell #1, having a pretilt angle of 2.0°, in the standard viewingdirection within the extent that did not pose any problem for real use.At a viewing angle of 60°, a decrease in luminance was evident in thestandard viewing direction to the extent unbearable for real use.

It can be concluded from Tables 1 to 7 that tone reversion can berestrained in the opposite viewing direction by adjusting the pretiltangle or the transmittance when white tone is being displayed. It can bealso concluded that in such an event, at a value ranging from 95% to 97%to which the ratio of the transmittance is normally set as thetransmittance for white tone, the setting of the pretilt angle in arange larger than 2° and smaller than 12° permits high quality images tobe displayed at a viewing angle of 50°0 with tone reversion beingrestrained in the opposite viewing direction and no decrease inluminance being observed in the standard viewing direction. It can befurther concluded that the setting of the pretilt angle in a range notless than 4° and not more than 10° permits high quality images to bedisplayed at a wide viewing angle of 70° with tone reversion beingrestrained in the opposite viewing direction and no decrease inluminance being observed in the standard viewing direction.

Moreover, it can be concluded that at a pretilt angle of 2° to 10°, towhich the pretilt angle is normally set, such setting that atransmittance not higher than 85% is derived as the transmittance whenwhite tone is being displayed permits high quality images to bedisplayed at a viewing angle of 50° with tone reversion being restrainedin the opposite viewing direction and no decrease in luminance beingobserved in the standard viewing direction. It can be also concludedthat such setting that a transmittance within a range not less than 90%and not more than 97% is derived, plus the adjustment of the pretiltangle, permits high quality images to be displayed at a wide viewingangle of 70° with tone reversion being restrained in the oppositeviewing direction and no decrease in luminance being observed in thestandard viewing direction.

Moreover, it can be concluded that a combination of the adjustment ofthe pretilt angle and that of the transmittance when white tone is beingdisplayed further enhances the effects of improvement.

Next, viewing angle dependency of the liquid crystal display device waschecked with the same samples #1 and #4 as above by using a measuringsystem including a light receiving element 21, an amplifier 22, and arecording device 23 as shown in FIG. 6.

In this measuring system, the liquid crystal cell 16 of the liquidcrystal display device is placed so that the surface 16 a facing theglass substrate 9 lies on the reference plane X-Y of the rectangularcoordinates XYZ. The light receiving element 21 is an element capable ofreceiving light at a certain solid light receiving angle, and is locateda predetermined distance away from the original point of the coordinatesat an angle (viewing angle) of φ with respect to the Z-directionorthogonal to the plane 16 a.

Upon measurement, monochromatic light having a wavelength of 550 nm isemitted from the surface opposite the plane 16 a to irradiate the liquidcrystal cell 16 in the measuring system. Part of the monochromatic lighthaving passed through the liquid crystal cell 16 enters the lightreceiving element 21. Output by the light receiving element 21 isamplified to a predetermined level by the amplifier 22, and recorded inthe recording device 23, such as a waveform memory or a recorder.

Here, the output level by the light receiving element 21 in response tothe applying of voltage to the sample cells #1 and #4 was measured withthe light receiving element 21 being fixed at a certain angle φ.

The measurement was done, assuming that the Y-direction is the left-handside of the screen and the X-direction is the downward direction(standard viewing direction) of the screen, while disposing the lightreceiving element 21 in the upward direction (opposite viewingdirection), the downward direction (standard viewing direction), and theright- and left-hand directions with the angle φ being maintained at50°.

Graphs in FIGS. 7(a) to 7(c) show results, illustrating the behavior ofthe light transmittances of the sample cells #4 and #1, havingrespective pretilt angles of 5.0° and 2.0°, in response to voltageapplied thereto, that is, the transmittance versus liquid crystalapplied voltage characteristics.

FIG. 7(a) shows results of the measurement from the upward direction inFIG. 2. FIG. 7(b) shows results of the measurement from the downwarddirection in FIG. 2. FIG. 7(c) shows results of the measurement from theright- and left-hand directions in FIG. 2.

Referring to FIG. 7(a), the curved alternative long and short dash lineL1 represents results of measurement in the front direction, i.e. thedirection normal to the surface. Both the sample cell #1 and the samplecell #4 exhibit the same transmittance versus liquid crystal appliedvoltage characteristics.

Referring to FIGS. 7(a) to 7(c), the solid lines L2, L4, and L6represent the sample cell #4, and the dotted lines L3, L5, and L7represent the sample #1.

To compare the sample cell #4 with the sample cell #1 in terms oftransmittance versus liquid crystal applied voltage characteristics inthe upward direction in FIG. 7(a), the curved line L3 for the samplecell #1 has a bumpy shape, or rise and fall of the transmittance,between about 1 V and 2 V. By contrast, the curved line L2 for thesample cell #4 is flat between about 1 V and 2 V with the transmittancestaying at a value, and has no bumpy shape, showing that the sample cell#4 is free from the tone reversion phenomenon.

To compare those sample cells in terms of transmittance versus liquidcrystal applied voltage characteristics in the downward, left-hand, andright-hand directions in FIGS. 7(b) and 7(c), the curved lines L4 and L6for the sample cell #4 and the curved lines L5 and L7 for the samplecell #1 show that the transmittance of the sample cell #4 drops a littlemore quickly than that of the sample cell #1. However, the transmittanceof the sample cell #4 starts to conform to that of the sample cell #1 ataround 2.5 V in FIG. 7(b) and at around 3 V in FIG. 7(c). Therefore, itcan be confirmed that the larger pretilt angle equalling 5.0° has noadverse effects.

The same results were obtained with sample cells prepared in the samemanner as the sample cells #1 to #7 except that those sample cells eachinclude optical retardation compensator plates 2 and 3 composed ofdiscotic liquid crystal treated with hybrid orientation on a transparentsupport base.

SECOND EXAMPLE

The present example is to substantiate the effects of the liquid crystaldisplay devices in accordance with the first to third embodiments. Here,three sample cells #16 to #18 were prepared by using Optomer AL (productname), available from Japan Synthetic Rubber Co., Ltd., as theorientation films 11 and 14 of the liquid crystal cell 16 of the liquidcrystal display device shown in FIG. 1, using as the liquid crystallayer 8 liquid crystal materials of which the pretilt angle is 3° and ofwhich the refractive index anisotropies Δn_(L)(550) at a wavelength of550 nm are 0.070, 0.080, and 0.095 respectively, and setting thethickness of the cells (of the liquid crystal layers 8) to 5 μm.

In the same manner as in the previous example, homogeneous cells wereprepared by injecting thereinto the materials for the sample cells #16to #18, and measured with a pretilt angle measuring device,NSMAP-3000LCD, for the pretilt angles of the sample cells #16 to #18.

Used as the optical retardation compensator plates 2 and 3 of the samplecells #16 to #18 are the optical retardation compensator plates 2 and 3of the same kind as those in the first example above including discoticliquid crystal treated with an oblique orientation technique.

The same measuring system as that in the first example above shown inFIG. 6 was used to measure the output level by the light receivingelement 21 in response to the applying of voltage to the sample cells#16 to #18 with the light receiving element 21 being fixed at a certainangle φ.

The measurement was done, assuming that the Y-direction is the left-handside of the screen and the X-direction is the downward direction(standard viewing direction) of the screen, while disposing the lightreceiving element 21 in the upward direction (opposite viewingdirection), the downward direction (standard viewing direction), and theright- and left-hand directions with the angle φ being maintained at50°.

Graphs in FIGS. 8(a) to 8(c) show results, illustrating the behavior oflight transmittance of the sample cells #16 to #18 in response tovoltage applied thereto, that is, the transmittance versus liquidcrystal applied voltage characteristics.

FIG. 8(a) shows results of the measurement from the upward direction inFIG. 2. FIG. 8(b) shows results of the measurement from the right-handdirection in FIG. 2. FIG. 8(c) shows results of the measurement from theleft-hand direction in FIG. 2.

Referring to FIGS. 8(a) to 8(c), the curved alternative long and shortdash lines L8, L11, and L4 represent the sample cell #16 using a liquidcrystal material of Δn_(L)(550)=0.070 for the liquid crystal layer 8,the solid lines L9, L12, and L15 represent the sample cell #17 using aliquid crystal material of Δn_(L)(550)=0.080 for the liquid crystallayer 8, and the dotted lines L10, L13, and L16 represent the samplecell #18 using a liquid crystal material of Δn_(L)(550)=0.095 for theliquid crystal layer 8.

Two comparative sample cells #103 and #104 were also prepared as acomparative example for the present example in the same manner as thesample cells of the present example except that those comparative samplecells use liquid crystal materials of which the refractive indexanisotropies Δn_(L)(550) at a wavelength of 550 nm are 0.060 and 0.120as the liquid crystal layer 8 of the liquid crystal cell 16 shown inFIG. 1. The measuring system shown in FIG. 6 was used to measure theoutput level by the light receiving element 21 in response to theapplying of voltage to the comparative sample cells #103 and #104 withthe light receiving element 21 being fixed at a certain angle φ in thesame manner as in the present example.

The measurement was done, assuming that the Y-direction is the left-handside of the screen and the X-direction is the downward direction(standard viewing direction) of the screen, while disposing the lightreceiving element 21 in the upward direction (opposite viewingdirection) and the right- and left-hand directions with the angle φbeing maintained at 50°.

Graphs in FIGS. 9(a) to 9(c) show results, illustrating the behavior oflight transmittance of the comparative sample cells #103 to #104 inresponse to voltage applied thereto, that is, the transmittance versusliquid crystal applied voltage characteristics.

FIG. 9(a) shows results of the measurement from the upward direction inFIG. 2. FIG. 9(b) shows results of the measurement from the right-handdirection in FIG. 2. FIG. 9(c) shows results of the measurement from theleft-hand direction in FIG. 2.

Referring to FIGS. 9(a) to 9(c), the solid curved lines L17, L19, andL21 represent the comparative sample cell #103 using a liquid crystalmaterial having Δn_(L)(550) of 0.060 for the liquid crystal layer 8, andthe dotted curved lines L18, L20, and L22 represent the comparativesample cell #104 using a liquid crystal material having Δn_(L)(550) of0.120 for the liquid crystal layer 8.

To compare the sample cells #16 to #18 and the comparative sample cells#103 and #104 in terms of transmittance versus liquid crystal appliedvoltage characteristics in the upward direction in FIGS. 8(a) and 9(a),the curved lines L9, L8, and L10 show that the transmittances drop bysufficient amounts with higher voltages. By contrast, in comparison withthe curved lines L8, L9, and L10, the curved line L18 shows that thetransmittance does not drop sufficiently with higher voltages, and thecurved line L17 shows that the transmittance drops and then rises withhigher voltages, resulting in tone reversion phenomenon.

To compare the sample cells #16 to #18 and the comparative sample cells#103 and #104 in terms of transmittance versus liquid crystal appliedvoltage characteristics in the right-hand direction in FIGS. 8(b) and9(b), the curved lines L11, L12, and L13 show that the transmittancesdrop almost to zero with higher voltages. The curved line L19 shows thatthe transmittance drops almost to zero with higher voltages as in FIG.8(b), while the curved line L20 shows that tone reversion phenomenonoccurs. The same results as in the right-hand direction were obtained inthe left-hand direction with the sample cells #16 to #18 and thecomparative sample cells #103 and #104.

Visual observations were conducted of the sample cells #16 to #18 andthe comparative sample cells #103 and #104 under white light.

The sample cells #16 to #18 and the comparative sample cell #103 showedcoloration in no direction at a viewing angle of 50°, displaying goodimages. By contrast, the comparative sample cell #104 showed colorationranging from yellow to orange in the right- and left-hand directions ata viewing angle of 50°.

It can be concluded from those results shown in FIGS. 8(a) to 8(c) thatif the liquid crystal layer 8 is made of a liquid crystal material ofwhich the refractive index anisotropy Δn_(L)(550) at a wavelength of 550nm is 0.070, 0.080, or 0.095, the transmittance drops by a sufficientamount with higher voltages, thereby shows no tone reversion phenomenon,expanding the effective viewing angle, and shows no colorationphenomenon, greatly improving the display quality of the liquid crystaldisplay device.

It can be concluded, on the other hand, from those results in FIGS. 9(a)to 9(c) that if the liquid crystal layer 8 is made of a liquid crystalmaterial of which the refractive index anisotropy Δn_(L)(550) at awavelength of 550 nm is 0.060 or 0.120, the viewing angle dependency isnot restrained satisfactorily.

The same results were obtained with sample cells and comparative samplecells prepared in the same manner as the sample cells #16 to #18 and thecomparative sample cells #103 and #104 except that those sample cellsand comparative sample cells include optical retardation compensatorplates 2 and 3 composed of discotic liquid crystal treated with hybridorientation on a transparent support base.

The transmittance versus liquid crystal applied voltage characteristicswere examined for the dependency thereof upon the inclination angle θ ofthe refractive index ellipsoid of the optical retardation compensatorplates 2 and 3, by varying the inclination angle θ. The results weresuch that the transmittance versus liquid crystal applied voltagecharacteristics remained virtually unchanged irrelevant to theorientation state of the discotic liquid crystal of the opticalretardation compensator plates 2 and 3, as long as the inclination angleθ stayed in the range of 15°≦θ≦75°. It was also observed that when theinclination angle θ was varied out of that range, the effective viewingangle did not become wider in the opposite viewing direction.

The transmittance versus liquid crystal applied voltage characteristicswere examined for the dependency thereof upon the second retardationvalue of the optical retardation compensator plates 2 and 3, by varyingthe second retardation value. The results were such that thetransmittance versus liquid crystal applied voltage characteristicsremained virtually unchanged irrelevant to the orientation state of thediscotic liquid crystal of the optical retardation compensator plates 2and 3, as long as the second retardation value stayed in the range of 80nm to 250 nm. It was also observed that when the second retardationvalue was varied out of that range, the effective viewing angle did notbecome wider in the opposite viewing direction.

In light of the results of the visual observations of the comparativesample cells #103 and #104, three sample cells #19 to #21 were preparedin the same manner as in the present example except that the samplecells #19 to #21 used liquid crystal materials of which the refractiveindex anisotropies Δn_(L)(550) at a wavelength of 550 nm are 0.065,0.100, and 0.115 as the liquid crystal layer 8 of the liquid crystalcell 16 shown in FIG. 1. The measuring system shown in FIG. 6 was usedto measure the output level by the light receiving element 21 inresponse to the applying of voltage to the sample cells #19 to #21 withthe light receiving element 21 being fixed at a certain angle φ in thesame manner as in the present example. Visual observations were alsoconducted of the sample cells #19 to #21 under white light.

The results show that the transmittance of the sample cell #20 with therefractive index anisotropy Δn_(L)(550) of 0.100 and that of the samplecell #21 with the refractive index anisotropy Δn_(L)(550) of 0.115 roseslightly with higher voltages in the right- and left-hand directionswith the angle φ of 50°. However, no tone reversion phenomenon wasvisually confirmed, and those rises in the transmittances were withinthe extent that did not pose any problem for real use. The results showno problem at all in the upward direction. Meanwhile, similarly to thetransmittance of the aforementioned comparative sample cell #103, thetransmittance of the sample cell #19 with the refractive indexanisotropy Δn_(L)(550) of 0.065 dropped slightly and then rose withhigher voltages in the upward direction. However, the rise in thetransmittance was relatively small as compared with that of thecomparative sample cell #103 shown in FIG. 9(a), being within the extentthat did not pose any problem for real use. The results show no problemat all in the right- and left-hand directions.

Visual observation discovered slight coloration ranging from yellow toorange with the sample cells #20 and #21, however, within the extentthat did not pose any problem for real use. Visual observation alsodiscovered slight bluish coloration with the sample cell #19, however,within the extent that did not pose any problem for real use.

As a supplement, the sample cell #19 and the comparative sample cell#103 were measured for transmittances when white tone was beingdisplayed in the direction normal to the surface of the liquid crystalcell 16, by applying a voltage of about 1 V. The results show that thetransmittance of the comparative sample cell #103 dropped to the extentunbearable for real use, while the transmittance of the sample cell #19dropped slightly, however, within the extent that did not pose anyproblem for real use.

The same results were obtained in a case where Optomer AL (productname), available from Japan Synthetic Rubber Co., Ltd., was used as theorientation films 11 and 14 of the liquid crystal cell 16 of the liquidcrystal display device shown in FIG. 1, and liquid crystal materialsthat formed pretilt angles of 4°, 5°, 10°, and 11° to the orientationfilms 11 and 14 were used as the liquid crystal layer 8.

THIRD EXAMPLE

The present example is to substantiate the effects of the liquid crystaldisplay device in accordance with the third embodiment. Here, fivesample cells #31 to #35 were prepared by using, as the liquid crystallayer 8 of the liquid crystal cell 16 of the liquid crystal displaydevice shown in FIG. 1, liquid crystal materials and optical retardationcompensator plates of which the relations described in the expression(1) were set to 0, 0.15, 0.25, 0.30, and 0.33 respectively, and settingthe thickness of the cells (of the liquid crystal layers 8) to 5 μm, therelation concerning the refractive index anisotropy Δn_(L)(450) of theliquid crystal layer 8 at a wavelength of 450 nm, the refractive indexanisotropy Δn_(L)(550) thereof at a wavelength of 550 nm, the refractiveindex anisotropy Δn_(F)(450) of the optical retardation compensatorplates 2 and 3 at a wavelength of 450 nm, the refractive indexanisotropy Δn_(F)(550) thereof at a wavelength of 550 nm.$\begin{matrix}\frac{\left( {\Delta\quad{{n_{L}(450)}/\Delta}\quad{n_{L}(550)}} \right) - 1}{\left( {\Delta\quad{{n_{F}(450)}/\Delta}\quad{n_{F}(550)}} \right) - 1} & (1)\end{matrix}$

Used as the optical retardation compensator plates 2 and 3 of the samplecells #31 to #35 are those constituted by a transparent support base(e.g., triacetylcellulose (TAC)) on which discotic liquid crystal isapplied. The discotic liquid crystal is treated with an obliqueorientation technique, and crosslinked. The optical retardationcompensator plates 2 and 3 each have resulting first and secondretardation values of 0 and 100 nm respectively, a principal refractiveindex nb inclining by 20° in the direction of arrow A with respect tothe z-coordinate axis of the x-, y-, and z-coordinates system, and aprincipal refractive index nc inclining by 20° in the direction of arrowB with respect to the x-coordinate axis (that is, the inclination angleof the refractive index ellipsoid θ=20°).

As a comparative example for the present example, comparative samplecells #301 to #303 were also prepared in the same manner as the presentexample except that liquid crystal materials and optical retardationcompensator plates of which the relations described in the expression(1) equaled 0.35, 1.0, and 1.1 were used as the liquid crystal layer 8of the liquid crystal cell 16 of the liquid crystal display device shownin FIG. 1.

Table 8 shows results of visual observations of the sample cells #31 to#35 and the comparative sample cells #301 to #303 under white light.

TABLE 8$\frac{{{{\Delta n}_{L}(450)}/{{\Delta n}_{L}(550)}} - 1}{{{{\Delta n}_{F}(450)}/{{\Delta n}_{F}(550)}} - 1}$Viewing 0 0.15 0.25 0.30 0.33 0.35 1.0 1.1 Angle (θ) #31 #32 #33 #34 #35#301 #302 #303 50° ∘ ∘ ∘ ∘ ∘ x x x 60° ∘ ∘ ∘ Δ x x x x 70° ∘ ∘ ∘ × x x xx (“∘” represents no coloration, “Δ” represents coloration within theextent that did not pose any problem for real use, and “x” representscoloration to the extent unbearable for real use.)

The sample cells #31 to #33 displayed good images with coloration beingobserved in no direction at a viewing angle of 70°. The sample cell #34displayed good images with coloration being observed in no direction atviewing angles up to 50°, however, displaying slight coloration withinthe extent that did not pose any problem for real use in the right- andleft-hand directions at a viewing angle of 60°. The sample cell #35displayed good images with coloration being observed in no direction atviewing angles up to 50°, however, displaying coloration to the extentunbearable for real use in the right- and left-hand directions at aviewing angle of 60°.

By contrast, the comparative sample cells #301 to #303 displayedcoloration ranging from yellow to orange to the extent unbearable forreal use in the right- and left-hand directions at a viewing angle aslow as 50°.

The same results were obtained with sample cells and comparative samplecells prepared in the same manner as the sample cells #31 to #35 and thecomparative sample cells #301 and #303 except that those sample cellsand comparative sample cells included optical retardation compensatorplates 2 and 3 composed of discotic liquid crystal treated with hybridorientation on a transparent support base.

FOURTH EXAMPLE

The present example is to substantiate the effects of the liquid crystaldisplay device in accordance with the third embodiment. Here, fivesample cells #41 to #45 were prepared by using, as the liquid crystallayer 8 of the liquid crystal cell 16 of the liquid crystal displaydevice shown in FIG. 1, liquid crystal materials and optical retardationcompensator plates of which the relations described in the expression(2) were set to 0, 0.10, 0.20, 0.23, and 0.25 respectively, and settingthe thickness of the cells (of the liquid crystal layers 8) to 5 μm, therelation concerning the refractive index anisotropy Δn_(L)(550) of theliquid crystal layer 8 at a wavelength of 550 nm, the refractive indexanisotropy Δn_(L)(650) thereof at a wavelength of 650 nm, the refractiveindex anisotropy Δn_(F)(550) of the optical retardation compensatorplates 2 and 3 at a wavelength of 550 nm, the refractive indexanisotropy Δn_(F)(650) thereof at a wavelength of 650 nm.$\begin{matrix}\frac{1 - \left( {\Delta\quad{{n_{L}(650)}/\Delta}\quad{n_{L}(550)}} \right)}{1 - \left( {\Delta\quad{{n_{F}(650)}/\Delta}\quad{n_{F}(550)}} \right)} & (2)\end{matrix}$

Used as the optical retardation compensator plates 2 and 3 of the samplecells #41 to #45 are those constituted by a transparent support base(e.g., triacetylcellulose (TAC)) on which discotic liquid crystal isapplied. The discotic liquid crystal is treated with an obliqueorientation technique, and crosslinked. The optical retardationcompensator plates 2 and 3 each have resulting first and secondretardation values of 0 and 100 nm respectively, a principal refractiveindex nb inclining by 20° in the direction of arrow A with respect tothe z-coordinate axis of the x-, y-, and z-coordinates system, and aprincipal refractive index nc inclining by 20° in the direction of arrowB with respect to the x-coordinate axis (that is, the inclination angleof the refractive index ellipsoid θ=20°).

As a comparative example for the present example, comparative samplecells #401 to #403 were also prepared in the same manner as the presentexample except that liquid crystal materials and optical retardationcompensator plates of which the relations described in the expression(2) equaled 0.27, 1.0, and 1.1 were used as the liquid crystal layer 8of the liquid crystal cell 16 of the liquid crystal display device shownin FIG. 1.

Table 9 shows results of visual observations of the sample cells #41 to#45 and the comparative sample cells #401 to #403 under white light.

TABLE 9$\frac{1 - {{{\Delta n}_{L}(450)}/{{\Delta n}_{L}(550)}}}{1 - {{{\Delta n}_{F}(450)}/{{\Delta n}_{F}(550)}}}$Viewing 0 0.10 0.20 0.23 0.25 0.27 1.0 1.1 Angle (θ) #41 #42 #43 #44 #45#401 #402 #403 50° ∘ ∘ ∘ ∘ ∘ x x x 60° ∘ ∘ ∘ Δ x x x x 70° ∘ ∘ ∘ × x x xx (“∘” represents no coloration, “Δ” represents coloration within theextent that did not pose any problem for real use, and “x” representscoloration to the extent unbearable for real use.)

The sample cells #41 to #43 displayed good images with coloration beingobserved in no direction at a viewing angle of 70°. The sample cell #44displayed good images with coloration being observed in no direction atviewing angles up to 50°, however displaying slight coloration withinthe extent that did not pose any problem for real use in the right- andleft-hand directions at a viewing angle of 60°. The sample cell #45displayed slight coloration within the extent that did not pose anyproblem for real use in the right- and left-hand directions at a viewingangle of 50°.

By contrast, the comparative sample cells #401 to #403 displayedcoloration ranging from yellow to orange to the extent unbearable forreal use in the right- and left-hand directions at a viewing angle aslow as 50°.

The same results were obtained with sample cells and comparative samplecells prepared in the same manner as the sample cells #41 to #45 and thecomparative sample cells #401 and #403 except that those sample cellsand comparative sample cells included optical retardation compensatorplates 2 and 3 composed of discotic liquid crystal treated with hybridorientation on a transparent support base.

As explained above, the first arrangement of the present inventionincorporates, between the liquid crystal layer and the polarizer, anoptical retardation compensator plate represented by a refractive indexellipsoid of which the three principal refractive indices, na, nb, andnc, are mutually related by the inequality na=nc>nb, and of which theshorter axis coincident with the principal refractive index nb inclineswith respect to the normal direction of the surface of the opticalretardation compensator plate. Therefore, with the arrangement, for acase where a linearly polarized ray is converted to an ellipticallypolarized ray according to the phase difference between the ordinary andextraordinary rays developed from the linearly polarized ray upon thepassing through the liquid crystal layer possessing birefringence, theoptical retardation compensator plate compensates for the phasedifference between the ordinary and extraordinary rays that variesdepending upon the viewing angle.

However, the compensation function of this kind still falls short ofsatisfying the increasing demand for a better restraint in the viewingangle dependency. Bearing that in mind, the inventors of the presentinvention have conducted further research diligently and found out thatthe pretilt angle formed by the orientation films and the longer axes ofliquid crystal molecules in the liquid crystal layer affects the tonereversion in the opposite viewing direction, especially, when halftoneis being displayed by applying to the liquid crystal a voltage that isclose to the threshold voltage for the liquid crystal, which has led tothe completion of the present invention.

With the liquid crystal display device of the first arrangement inaccordance with the present invention, the pretilt angle of the liquidcrystal layer sealed in the liquid crystal display element is set withinsuch a range that tone reversion does not occur in the opposite viewingdirection when halftone is being displayed by applying to the liquidcrystal a voltage that is close to the threshold voltage for the liquidcrystal. This can eliminate the tone reversion in the opposite viewingdirection on a screen displaying halftone, and thereby further restrainthe viewing angle dependency of the screen. The contrast variations andcoloration are also restrained better than only by the compensationfunction by the optical retardation compensator plate.

The inventors have found that the larger the pretilt angles are, theless likely the tone reversion occurs in the opposite viewing directionwhen halftone is being displayed by applying to the liquid crystal avoltage that is close to the threshold voltage for the liquid crystal.However, the inventors have also found that too large pretilt anglescause an abrupt decrease in luminance in the standard viewing directionwhen halftone is being displayed. Thus, in the first arrangement above,the abrupt decrease in luminance can be restrained in the standardviewing direction when halftone is being displayed, by further settingthe pretilt angle within such a range that luminance does not decreaseabruptly in the standard viewing direction when halftone is beingdisplayed by applying to the liquid crystal a voltage that is close tothe threshold voltage for the liquid crystal.

Specifically, the range that does not cause tone reversion in theopposite viewing direction when halftone is being displayed by applyingto the liquid crystal a voltage that is close to the threshold voltagefor the liquid crystal, and that does not cause an abrupt decrease inluminance in the standard viewing direction when halftone is beingdisplayed refers to the setting of the pretilt angle within a rangelarger than 2° and smaller than 12°.

Although in some instances still incapable of completely eliminatingtone reversion in the opposite viewing direction at the viewing angle of50°, which is the viewing angle typically required for liquid crystaldisplay devices, the setting enables the liquid crystal display deviceto be viewed in every direction without problems for real use.

The above range, for a case of liquid crystal display devices with widerviewing angles such as 70°, refers to the setting of the pretilt anglewithin a range not smaller than 4° and not larger than 10°.

The setting enables the liquid crystal display device to be free fromtone reversion in the opposite viewing direction at the viewing angle of70°, which is the viewing angle typically required for the liquidcrystal display device with a wider viewing angle, when halftone isbeing displayed.

For these reasons, with the first arrangement, the contrast ratio inblack and white display is not affected by the viewing angle of theobserver, and the quality of images displayed by the liquid crystaldisplay device is greatly improved.

As explained above, even if a linearly polarized ray is converted to anelliptically polarized ray according to the phase difference between theordinary and extraordinary rays developed from the linearly polarizedray upon the passing through the liquid crystal layer possessingbirefringence, the second arrangement, similarly to the firstarrangement, compensates for the phase difference by the opticalretardation compensator plate.

However, the compensation function of this kind still falls short ofsatisfying the increasing demand for a better restraint in the viewingangle dependency. Bearing that in mind, the inventors of the presentinvention have conducted further research diligently and found out thatthe value of applied voltage for displaying halftone obtained byapplying to the liquid crystal a voltage that is close to the thresholdvoltage for the liquid crystal affects the tone reversion in theopposite viewing direction when halftone is being displayed, which hasled to the completion of the present invention.

With the liquid crystal display device of the second arrangement inaccordance with the present invention, the value of applied voltage fordisplaying halftone obtained by applying to the liquid crystal a voltagethat is close to the threshold voltage for the liquid crystal is setwithin such a range that tone reversion does not occur in the oppositeviewing direction when halftone is being displayed. This can eliminatethe tone reversion in the opposite viewing direction with a screendisplaying halftone, and thereby further restrain the viewing angledependency of the screen. The contrast variations and coloration arealso restrained better than only by the compensation function by theoptical retardation compensator plate.

The voltage for displaying halftone is set in the Normally White mode,as an example, by way of the ratio of the transmittance for the whitetone to the transmittance for the OFF state. The inventors have foundthat the lower the transmittance is, the less likely the tone reversionoccurs in the opposite viewing direction when white tone is beingdisplayed. However, the inventors have also found that too lowtransmittances cause an abrupt decrease in luminance in the standardviewing direction. Thus, in the second arrangement above, the abruptdecrease in luminance can be restrained in the standard viewingdirection when halftone is being displayed, by further setting the valueof applied voltage for displaying halftone obtained by applying to theliquid crystal a voltage that is close to the threshold voltage for theliquid crystal within such a range that luminance does not decreaseabruptly in the standard viewing direction when halftone is beingdisplayed.

Specifically, the range that does not cause tone reversion in theopposite viewing direction when halftone is being displayed by applyingto the liquid crystal a voltage that is close to the threshold voltagefor the liquid crystal, and that does not cause an abrupt decrease inluminance in the standard viewing direction when halftone is beingdisplayed refers to the setting of the value of applied voltage fordisplaying halftone obtained by applying to the liquid crystal a voltagethat is close to the threshold voltage for the liquid crystal so as toobtain a transmittance higher than 85% that in a bright state (OFFstate) where no voltage is applied to the liquid crystal.

Although in some instances still incapable of completely eliminatingtone reversion in the opposite viewing direction at the viewing angle of50°, which is the viewing angle typically required for liquid crystaldisplay devices, the setting enables the liquid crystal display deviceto be viewed in every direction without problems for real use.

The above range, for a case of liquid crystal display devices with widerviewing angles such as 70°, refers to the setting of the value ofapplied voltage for displaying halftone obtained by applying to theliquid crystal a voltage that is close to the threshold voltage for theliquid crystal so as to obtain a transmittance within a range not lessthan 90% and not more than 97% that in a bright state (OFF state) whereno voltage is applied to the liquid crystal.

The setting enables the liquid crystal display device to be free fromtone reversion in the opposite viewing direction at the viewing angle of70°, which is the viewing angle typically required for the liquidcrystal display device with a wider viewing angle, when halftone isbeing displayed.

For these reasons, with the arrangement, the contrast ratio in black andwhite display is not affected by the viewing angle of the observer, andthe quality of images displayed by the liquid crystal display device isgreatly improved.

As explained above, even if a linearly polarized ray is converted to anelliptically polarized ray according to the phase difference between theordinary and extraordinary rays developed from the linearly polarizedray upon the passing through the liquid crystal layer possessingbirefringence, the arrangement, similarly to the first arrangement,compensates for the phase difference by the optical retardationcompensator plate.

However, the compensation function of this kind still falls short ofsatisfying the increasing demand for a better restraint in the viewingangle dependency. Bearing that in mind, the inventors of the presentinvention have conducted further research diligently and found out thatthe ratios of the variation in the refractive index anisotropy, Δn_(L),of the liquid crystal material for the liquid crystal layer with thewavelength of light and of the variation in the refractive indexanisotropy, Δn_(F), of the optical retardation compensator plate withthe wavelength of light affect the coloration on the liquid crystalscreen depending upon the viewing angle, which has led to the completionof the present invention.

With the liquid crystal display device of the third arrangement inaccordance with the present invention, the ratios of the variation inthe refractive index anisotropy, Δn_(L), of the liquid crystal materialfor the liquid crystal layer with the wavelength of light and of thevariation in the refractive index anisotropy, Δn_(F), of the opticalretardation compensator plate with the wavelength of light are setwithin such a range that viewing angle dependency does not causecoloration on the liquid crystal screen. This can further restraincoloration on the screen. The contrast variations and tone reversion arealso restrained better than only by the compensation function by theoptical retardation compensator plate.

The range that does not cause coloration on the liquid crystal screendepending upon the viewing angle of the above ratio is the rangesatisfying the inequality above.

Specifically, as described above, the range refers to the setting of theratio, Δn_(L)(450)/Δn_(L)(550), of the refractive index anisotropy,Δn_(L)(450), of the liquid crystal material for the liquid crystal layerto light having a wavelength of 450 nm and the refractive indexanisotropy, Δn_(L)(550), thereof to light having a wavelength of 550 nm,and the ratio, Δn_(F)(450)/Δn_(F)(550), of the refractive indexanisotropy, Δn_(F)(450), of the optical retardation compensator plate tolight having a wavelength of 450 nm and the refractive index anisotropy,Δn_(F)(550), thereof to light having a wavelength of 550 nm so as tosatisfy the inequality:$0 \leq \frac{\left( {\Delta\quad{{n_{L}(450)}/\Delta}\quad{n_{L}(550)}} \right) - 1}{\left( {\Delta\quad{{n_{F}(450)}/\Delta}\quad{n_{F}(550)}} \right) - 1} < 0.35$

Alternatively, as described above, the range refers to the setting ofthe ratio, Δn_(L)(650)/Δn_(L)(550), of the refractive index anisotropy,Δn_(L)(650), of the liquid crystal material for the liquid crystal layerto light having a wavelength of 650 nm and the refractive indexanisotropy, Δn_(L)(550), thereof to light having a wavelength of 550 nm,and the ratio, Δn_(F)(650)/Δn_(F)(550), of the refractive indexanisotropy, Δn_(F)(650), of the optical retardation compensator plate tolight having a wavelength of 650 nm and the refractive index anisotropy,Δn_(F)(550), thereof to light having a wavelength of 550 nm so as tosatisfy the inequality:$0 \leq \frac{1 - \left( {\Delta\quad{{n_{L}(650)}/\Delta}\quad{n_{L}(550)}} \right)}{1 - \left( {\Delta\quad{{n_{F}(650)}/\Delta}\quad{n_{F}(550)}} \right)} < 0.27$

Although in some instances still incapable of completely eliminatingcoloration at the viewing angle of 50°, which is the viewing angletypically required for liquid crystal display devices, the setting ofthe ratio to fall within either of the above ranges enables the liquidcrystal display device to be viewed in every direction without problemsfor real use.

The above ranges of the ratio, for a case of liquid crystal displaydevices with wider viewing angles such as 70°, are preferably the rangessatisfying the inequality above.

That is, as described above, the range refers to the setting of theratio so as to satisfy the inequality:$0 \leq \frac{\left( {\Delta\quad{{n_{L}(450)}/\Delta}\quad{n_{L}(550)}} \right) - 1}{\left( {\Delta\quad{{n_{F}(450)}/\Delta}\quad{n_{F}(550)}} \right) - 1} \leq 0.25$

Alternatively, as described above, the range refers to the setting ofthe ratio so as to satisfy the inequality:$0 \leq \frac{1 - \left( {\Delta\quad{{n_{L}(650)}/\Delta}\quad{n_{L}(550)}} \right)}{1 - \left( {\Delta\quad{{n_{F}(650)}/\Delta}\quad{n_{F}(550)}} \right)} \leq 0.20$

The setting described above enables the liquid crystal display device tobe completely free from coloration phenomenon for every direction at theviewing angle of 70°, which is the viewing angle typically required forthe liquid crystal display device with a wider viewing angle.

Moreover, as described above, in the first, second, and thirdarrangements, the liquid crystal display device in accordance with thepresent invention is preferably arranged so that the refractive indexanisotropy, Δn_(L)(550), of the liquid crystal material for the liquidcrystal layer to light having a wavelength of 550 nm is set within arange larger than 0.060 and smaller than 0.120.

This is because of a confirmation that if the refractive indexanisotropy, Δn_(L)(550), of the liquid crystal material to light havinga wavelength of 550 nm, which is approximately the mid-range of thevisible region of the spectrum, is either not larger than 0.060 or notsmaller than 0.120, tone reversion phenomenon and/or a decrease incontrast ratio occur(s) depending upon the viewing direction. Therefore,the phase difference that occurs to the liquid crystal display elementin accordance with the viewing angle can be eliminated by setting therefractive index anisotropy, Δn_(L)(550), of the liquid crystal materialto light having a wavelength of 550 nm so as to be within a range largerthan 0.060 and smaller than 0.120. This can further restrain thecontrast variations and tone reversion phenomenon in the right- andleft-hand directions, as well as the coloration phenomenon that occursdepending upon the viewing angle.

In such an event the phase difference that occurs to the liquid crystaldisplay element in accordance with the viewing angle can be moreeffectively eliminated by setting the refractive index anisotropy,Δn_(L)(550), of the liquid crystal material for the liquid crystal layerto light having a wavelength of 550 nm so as to be within a range notsmaller than 0.070 and not larger than 0.095. This can surely restrainthe contrast variations and tone reversion phenomenon in the right- andleft-hand directions of the images displayed by the liquid crystaldisplay device.

Moreover, as described above, in the first, second, and thirdarrangements, the liquid crystal display device in accordance with theinvention is preferably arranged so that the or each optical retardationcompensator plate is represented by a refractive index ellipsoidinclining by an inclination angle set within a range of 15° to 75°.

By setting the inclination angle of the refractive index ellipsoid to bewithin a range of 15° to 75° with respect to the or each opticalretardation compensator plate incorporated in the liquid crystal displaydevice, it is assured that the present invention provides theaforementioned compensation function for the phase difference by theoptical retardation compensator plate.

Moreover, as described above, in the first, second, and thirdarrangements, the liquid crystal display device in accordance with theinvention is preferably arranged so that the or each optical retardationcompensator plate has a product, (n_(a)−n_(b))×d, of the differencebetween the principal refractive indices, na and nb, and the thickness,d, of the optical retardation compensator plate, the product being setto be from 80 nm to 250 nm.

By setting the product, (n_(a)−n_(b))×d, of the difference between theprincipal refractive indices, na and nb, and the thickness, d, of theoptical retardation compensator plate, so as to be from 80 nm to 250 nmwith respect to the or each optical retardation compensator plateincorporated in the liquid crystal display device, it is assured thatthe present invention provides the aforementioned compensation functionfor the phase difference by the optical retardation compensator plate.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art intended tobe included within the scope of the following claims.

1. A liquid crystal display device, comprising: a liquid crystal display element formed so that a liquid crystal layer is disposed between a pair of substrates; a pair of polarizers disposed so as to flank said liquid crystal display element; an orientation film provided on each of the substrates for orienting the liquid crystal layer, each said orientation film being rubbed parallel to an absorption axis of the polarizer plate provided on the same side of the device as the substrate upon which that orientation film is provided; and at least one optical retardation compensator plate disposed between said liquid crystal display element and said polarizers, said at least one optical retardation compensator plate being represented by an inclining refractive ellipsoid, wherein the refractive index ellipsoid has three principal refractive indices, n_(a), n_(b), and n_(c), each of n_(a) and n_(c) is greater than n_(b), and the refractive index ellipsoid inclines as the direction of the principal refractive index n_(b) inclines to the normal to the surface of said at least one optical retardation compensator plate about the direction of the principal refractive index n_(a) or n_(c) in said surface; wherein a value of applied voltage for displaying halftone obtained by applying to said liquid crystal display layer a voltage that is close to the threshold voltage of said liquid crystal layer is set within such a range that tone reversion does not occur at at least 50° from the normal to a surface of the liquid crystal display when halftone is being displayed thereby, and, wherein the value of the applied voltage is set so that a transmittance when white tone is being displayed is in a range not less than 90% and not more than 97% of the transmittance in a bright state where no voltage is being applied to the liquid crystal.
 2. The liquid crystal display device as defined in claim 1, wherein the refractive index ellipsoid has three principal refractive indices, n_(a), n_(b), and n_(c), mutually related by the inequality n_(a)=n_(c)>n_(b), and inclines as the direction of the principal refractive index n_(b) inclines from the normal direction to the surface of said at least one optical retardation compensator plate around the direction of the principal refractive index n_(a) or n_(c) disposed in said surface.
 3. The liquid crystal display device as defined in claim 1, wherein the value of the applied voltage is further set within a range that gray-scale inversion does not occur in the standard viewing direction when halftone is being displayed, and wherein the value of the applied voltage is set to obtain a transmittance higher than 85% of the transmittance in a bright state where no voltage is applied to the liquid crystal layer.
 4. The liquid crystal display device as defined in claim 2, wherein the value of the applied voltage is set to obtain a transmittance within a range not less than 90% and not more than 97% of the transmittance in a bright state where no voltage is applied to the liquid crystal layer.
 5. A liquid crystal display device, comprising: a liquid crystal display element formed so that a liquid crystal layer is disposed between a pair of substrates; a pair of polarizers disposed so as to flank said liquid crystal display element; an orientation film provided on each of the substrates for orienting the liquid crystal layer, each said orientation film being rubbed parallel to an absorption axis of the polarizer plate provided on the same side of the device as the substrate upon which that orientation film is provided; and at least one optical retardation compensator plate disposed between said liquid crystal display element and said polarizers, said at least one optical retardation compensator plate being represented by an inclining refractive ellipsoid, wherein the refractive index ellipsoid has three principal refractive indices, n_(a), n_(b), and n_(c), each of n_(a) and n_(c) is greater than n_(b), and the refractive index ellipsoid inclines as the direction of the principal refractive index n_(b) inclines to the normal to the surface of said at least one optical retardation compensator plate about the direction of the principal refractive index n_(a) or n_(c) in said surface; wherein a value of applied voltage for displaying halftone obtained by applying to said liquid crystal display layer a voltage that is close to the threshold voltage of said liquid crystal layer is set within such a range that tone reversion does not occur at at least 60° from the normal to a surface of the liquid crystal display when halftone is being displayed thereby and, wherein the value of the applied voltage is set so that a transmittance when white tone is being displayed is in a range not less than 90% and not more than 97% of the transmittance in a bright state where no voltage is being applied to the liquid crystal.
 6. The liquid crystal display device as defined in claim 5, wherein the refractive index ellipsoid has three principal refractive indices, n_(a), n_(b), and n_(c), mutually related by the inequality n_(a)=n_(c)>n_(b), and inclines as the direction of the principal refractive index n_(b) inclines from the normal direction to the surface of said at least one optical retardation compensator plate around the direction of the principal refractive index n_(a) or n_(c) disposed in said surface.
 7. The liquid crystal display device as defined in claim 5, wherein the value of the applied voltage is further set within a range that gray-scale inversion does not occur in the standard viewing direction when halftone is being displayed, and wherein the value of the applied voltage is set to obtain a transmittance higher than 85% of the transmittance in a bright state where no voltage is applied to the liquid crystal layer.
 8. The liquid crystal display device as defined in claim 7, wherein the value of the applied voltage is set to obtain a transmittance within a range not less than 90% and not more than 97% of the transmittance in a bright state where no voltage is applied to the liquid crystal layer.
 9. A liquid crystal display device, comprising: a liquid crystal display element formed so that a liquid crystal layer is disposed between a pair of substrates; a pair of polarizers disposed so as to flank said liquid crystal display element; an orientation film provided on each of the substrates for orienting the liquid crystal layer, each orientation film being rubbed parallel to an absorption axis of the polarizer plate provided on the same side of the device as the substrate upon which that orientation film is provided; and at least one optical retardation compensator plate disposed between said liquid crystal display element and said polarizers, said at least one optical retardation compensator plate being represented by an inclining refractive ellipsoid, wherein the refractive index ellipsoid has three principal refractive indices, n_(a), n_(b), and n_(c), each of n_(a) and n_(c) is greater than n_(b), and the refractive index ellipsoid inclines as the direction of the principal refractive index n_(b) inclines to the normal to the surface of said at least one optical retardation compensator plate about the direction of the principal refractive index n_(a) or n_(c) in said surface; wherein a value of applied voltage for displaying halftone obtained by applying to said liquid crystal display layer a voltage that is close to the threshold voltage of said liquid crystal layer is set within such a range that tone reversion does not occur at at least 70° from the normal to a surface of the liquid crystal display when halftone is being displayed thereby, and wherein the value of applied voltage is set so that a transmittance when white tone is being displayed is in a range not less than 90% and not more than 97% of the transmittance in a bright state where no voltage is being applied to the liquid crystal.
 10. The liquid crystal display device as defined in claim 9, wherein the refractive index ellipsoid has three principal refractive indices, n_(a), n_(b), and n_(c), mutually related by the inequality n_(a)=n_(c)>n_(b), and inclines as the direction of the principal refractive index n_(b) inclines from the normal direction to the surface of said at least one optical retardation compensator plate around the direction of the principal refractive index n_(a) or n_(c) disposed in said surface.
 11. The liquid crystal display device as defined in claim 9, wherein the value of the applied voltage is further set within a range that gray-scale inversion does not occur in the standard viewing direction when halftone is being displayed, and wherein the value of the applied voltage is set to obtain a transmittance higher than 85% of the transmittance in a bright state where no voltage is applied to the liquid crystal layer.
 12. The liquid crystal display device as defined in claim 11, wherein the value of the applied voltage is set to obtain a transmittance within a range not less than 90% and not more than 97% of the transmittance in a bright state where no voltage is applied to the liquid crystal layer.
 13. A liquid crystal display device, comprising: a liquid crystal display element formed so that a liquid crystal layer is disposed between a pair of substrates; a pair of polarizers disposed so as to flank said liquid crystal display element; an orientation film provided on each of the substrates for orienting the liquid crystal layer, each orientation film being rubbed parallel to an absorption axis of the polarizer plate provided on the same side of the device as the substrate upon which that orientation film is provided; and at least one optical retardation compensator plate being represented by an inclining refractive index ellipsoid, wherein the refractive index ellipsoid has three principal refractive indices, n_(a), n_(b), and n_(c), each of n_(a) and n_(c) is treater than n_(b), and the refractive index ellipsoid inclines as the direction of the principal refractive index n_(b) inclines to the normal to the surface of said at least one optical retardation compensator plate about the direction of the principal refractive index n_(a) or n_(c) in said surface thereof; wherein a value of the applied voltage for displaying halftone is obtained by applying to said liquid crystal layer a voltage that is close to the threshold voltage of said liquid crystal layer is set, and wherein the value of the applied voltage is set to obtain a transmittance within a range not less than 90% and not more than 97% of the transmittance in a bright state where no voltage is applied to the liquid crystal layer.
 14. The liquid crystal display device as defined in claim 13, wherein said refractive index ellipsoid has three principal refractive indices, n_(a), n_(b), and n_(c), mutually related by the inequality n_(a)=n_(c)>n_(b), and inclines as the direction of the principal refractive index n_(b) inclines from the normal direction to the surface of said at least one optical retardation compensator plate around the direction of the principal refractive index n_(a) or n_(c) disposed in said surface.
 15. The liquid crystal display device as defined in claim 13, wherein the refractive index anisotropy, Δn_(L)(550), of a liquid crystal material for the liquid crystal layer to light having a wavelength of 550 nm is set within a range larger than 0.060 and smaller than 0.120.
 16. The liquid crystal display device as defined in claim 15, wherein Δn_(L)(550) is set within a range larger than 0.070 and smaller than 0.095.
 17. The liquid crystal display device as defined in claim 13, wherein the refractive index ellipsoid inclines by an inclination angle set within a range of 15° to 75°.
 18. The liquid crystal display device as defined in claim 13, wherein said at least one optical retardation compensator plate has a product (n_(a)−n_(b))×d, of the difference between the principal refractive indices, n_(a) and n_(b), and the thickness, d, of the optical retardation compensator plate, the product being set to be from 80 nm to 250 nm. 